Add custom response headers for backend service

[cagataygurturk]

Part of hashicorp/terraform-provider-google#7299

First PR to magic-modules. Please forgive rookie mistakes.

I am unable to run the acceptance tests also.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: added `custom_response_headers` field to `google_compute_backend_service` resource

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@ndmckinley, please review this PR or find an appropriate assignee.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 45 insertions(+))
Terraform Beta: Diff ( 4 files changed, 45 insertions(+))
Ansible: Diff ( 2 files changed, 19 insertions(+))
TF Conversion: Diff ( 1 file changed, 11 insertions(+))
TF OiCS: Diff ( 1 file changed, 1 insertion(+))
Inspec: Diff ( 4 files changed, 7 insertions(+))

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[nat-henderson]

Looks great to me! Let me just run those tests and reconfirm that the changelog is set up right...

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=158180"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 45 insertions(+))
Terraform Beta: Diff ( 4 files changed, 45 insertions(+))
Ansible: Diff ( 2 files changed, 19 insertions(+))
TF Conversion: Diff ( 1 file changed, 11 insertions(+))
TF OiCS: Diff ( 1 file changed, 1 insertion(+))
Inspec: Diff ( 4 files changed, 7 insertions(+))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceCloudIdentityGroupMemberships_basic|TestAccDataSourceSpannerInstance_basic|TestAccApiGatewayApiConfigIamBindingGenerated|TestAccApiGatewayApiConfigIamMemberGenerated|TestAccApiGatewayApiConfigIamPolicyGenerated|TestAccApiGatewayGatewayIamBindingGenerated|TestAccApiGatewayGatewayIamMemberGenerated|TestAccApiGatewayGatewayIamPolicyGenerated|TestAccApiGatewayApiConfig_apigatewayApiConfigBasicExampleUpdated|TestAccApiGatewayApiConfig_apigatewayApiConfigBasicExample|TestAccApiGatewayGateway_apigatewayGatewayBasicExample|TestAccApiGatewayGateway_apigatewayGatewayBasicExampleUpdated|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccBigqueryDataTransferConfig|TestAccBigtableAppProfile_bigtableAppProfileMulticlusterExample|TestAccBigtableAppProfile_bigtableAppProfileSingleclusterExample|TestAccCloudIdentityGroup_cloudIdentityGroupsBasicExample|TestAccCloudIdentityGroupMembership_cloudIdentityGroupMembershipExample|TestAccCloudIdentityGroupMembership_cloudIdentityGroupMembershipUserExample|TestAccComposerEnvironment_withUpdateOnCreate|TestAccComputeBackendService_backendServiceNetworkEndpointExample|TestAccComputeBackendService_updatePreservesOptionalParameters|TestAccComputeBackendService_basic|TestAccComputeBackendService_withBackend|TestAccComputeBackendService_withBackendAndIAP|TestAccComputeBackendService_withConnectionDrainingAndUpdate|TestAccComputeBackendService_withSecurityPolicy|TestAccComputeBackendService_withBackendAndMaxUtilization|TestAccComputeBackendService_withCustomHeaders|TestAccComputeBackendService_withSessionAffinity|TestAccComputeBackendService_withMaxRatePerEndpoint|TestAccComputeBackendService_withMaxConnections|TestAccComputeBackendService_withMaxConnectionsPerEndpoint|TestAccComputeBackendService_withMaxConnectionsPerInstance|TestAccComputeBackendService_withLogConfig|TestAccComputeBackendService_trafficDirectorUpdateBasic|TestAccComputeBackendService_trafficDirectorUpdateFull|TestAccComputeInstanceGroup_rename|TestAccFilestoreInstance_filestoreInstanceBasicExample|TestAccFilestoreInstance_update|TestAccLoggingBillingAccountSink_update|TestAccLoggingBillingAccountSink_heredoc|TestAccLoggingFolderSink_heredoc|TestAccLoggingOrganizationSink_heredoc|TestAccLoggingProjectSink_heredoc|TestAccOSLoginSSHPublicKey_osLoginSshKeyExpiry|TestAccStorageBucket_cors You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=158181"

[nat-henderson]

Tests pass - merging.

[nat-henderson]

Thanks for contributing! :)