feat(open-api-gateway): support singular tags on operations

[cogwirrel]

This change adds support for tagging operations, for example:

  /hello:
    get:
      tags:
        - foo
      operationId: sayHello
      responses:
        '200':
          description: Ok

Previously, tagging would cause broken lambda handler wrappers etc to be generated since the CDK construct assumes a single OperationConfig, and tagging APIs caused multiple to be generated.

In this change, we always generate a single OperationConfig, but iterate over all apis to make sure all operations for all tags are included.

⚠️ Note that this change only supports tagging each operation with zero or one tags, multiple tags will cause duplicated code to be generated which will not build - support for multiple tags requires an upstream change in openapi-generator: OpenAPITools/openapi-generator#14568

We change the default configuration for the Smithy openapi projection to preserve tags since if they are specified in the model users would expect them to apply to the generated code/documentation etc. This means that operations can be tagged in the Smithy model like so:

@readonly
@http(method: "GET", uri: "/hello")
@tags(["foo"])
operation SayHello {
    input: SayHelloInput
    output: SayHelloOutput
    errors: [ApiError]
}

This behaviour can be overridden in the project's smithyBuildOptions, ie:

const api = new SmithyApiGatewayTsProject({
  ...
  smithyBuildOptions: {
    projections: {
      openapi: {
        plugins: {
          openapi: {
            tags: false,
          },
        },
      },
    },
  },
});

re #269

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[JeremyJonas]

Looks awesome, great work adding the validation and actionable error handling :D