[Azure Search] Correct the definition of WebApiSkill

[arv100kri]

Breaking change notice: This PR fixes the incorrect definition of WebApiSkill in swagger.
When it was first introduced, the swagger definition incorrectly listed webApiHttpHeaders as a complex property with a nested headers property.

This resulted in a 400 bad request, when attempting to create a custom web api skill using the SDK.
This commit fixes that, and a subsequent (also breaking) change in the SDK will fix the model and add test coverage.

Latest improvements:

MSFT employees can try out our new experience at OpenAPI Hub - one location for using our validation tools and finding your workflow.

Contribution checklist:

• I have reviewed the documentation for the workflow.
• Validation tools were run on swagger spec(s) and have all been fixed in this PR.
• The OpenAPI Hub was used for checking validation status and next steps.

ARM API Review Checklist

• Service team MUST add the "WaitForARMFeedback" label if the management plane API changes fall into one of the below categories.
• adding/removing APIs.
• adding/removing properties.
• adding/removing API-version.
• adding a new service in Azure.

Failure to comply may result in delays for manifest application. Note this does not apply to data plane APIs.

• If you are blocked on ARM review and want to get the PR merged urgently, please get the ARM oncall for reviews (RP Manifest Approvers team under Azure Resource Manager service) from IcM and reach out to them.
  Please follow the link to find more details on API review process.

[openapi-sdkautomation]

SDK Automation [Logs] (Generated from 3752360, Iteration 4)

No readme.md specification configuration files were found that are associated with the files modified in this pull request.

[AutorestCI]

Automation for azure-sdk-for-go

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[AutorestCI]

Automation for azure-sdk-for-python

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[azuresdkci]

Can one of the admins verify this patch?