Promote CloudBuildSource to v1 API

[danyinggu]

Helps with #1413

Proposed Changes

• Promote CloudBuildSource API to v1.

Release Note

Add v1 API for CloudBuildSource.

Docs

[capri-xiyue]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: capri-xiyue, danyinggu

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• OWNERS [capri-xiyue]

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-google-knative-gcp-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/events/v1/cloudbuildsource_defaults.go	Do not exist	100.0%
pkg/apis/events/v1/cloudbuildsource_lifecycle.go	Do not exist	100.0%
pkg/apis/events/v1/cloudbuildsource_types.go	Do not exist	75.0%
pkg/apis/events/v1/cloudbuildsource_validation.go	Do not exist	75.0%

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-google-knative-gcp-wi-tests	2020-09-01 20:07:25.341 +0000 UTC	1/3

Automatically retrying due to test flakiness...
/test pull-google-knative-gcp-wi-tests

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-google-knative-gcp-integration-tests	2020-09-01 20:07:25.346 +0000 UTC	1/3

Automatically retrying due to test flakiness...
/test pull-google-knative-gcp-integration-tests