Maps all unique content-types for operations and set passthroughBehavior to never in CORS preflight integration

[jfkisafk]

Background

• What do these changes do?
  • Maps all unique content-types for operations in CORS preflight integration template, in addition to the application/json template that is currently added.
  • Sets the passthroughBehavior for the mock integration to "never" to fail-fast with 415 Unsupported in case a request is made with unsupported Content-Type.
  • Both these features are behind a new OpenAPI configuration syncCorsPreflightIntegration (not sure about the name), which the service teams can turn on.
• Why are they important?
  • So that API Gateway does not error out when OPTIONS request is made with a custom payload and a Content-Type other than application/json. This will pass the request to the actual GET/PUT/... method.
  • Requests made with unsupported Content-Type fail fast with 415 Unsupported instead of receiving 500 InternalServerError from API Gateway.

Testing

• How did you test these changes?
  • [x] Added test cases for the new changes.
  • [x] Checked backwards compatibility by running ./gradlew clean build.

Links

• closes smithy-lang/smithy#2289
• https://docs.aws.amazon.com/apigateway/latest/developerguide/integration-passthrough-behaviors.html

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jfkisafk]

Please lmk if this feature should be launched behind a feature flag in openapi settings or if I need to update the docs as well?