Add additional default CORS headers with config

[srchase]

Adds additional CORS headers when converting to an OpenAPI model for Amazon API Gateway.

By default, amz-sdk-invocation-id and amz-sdk-request headers will be added to the CORS preflight request.

On the openapi configuration, the additionalAllowedCorsHeaders option can be used to set other additional headers in place of these default headers, or to omit these default headers entirely by setting an empty array.

Sample config:

{
    "version": "1.0",
    "plugins": {
        "openapi": {
            "service": "smithy.example#Weather",
            "additionalAllowedCorsHeaders": ["foo-header", "bar-header"]
        }
    }
}

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