(apigateway): Add support for importing existing Cognito user pool authorizers

Add support to import existing Cognito user pool authorizers, so I could use `aws_apigateway.Method` instead of `aws_apigateway.CfnMethod`.

### Use Case
I have multiple stacks using same API gateway. Base stack creates the authorizer and exports authorizer ID.
Other stacks should be able to use something like `apigateway.CognitoUserPoolsAuthorizer.from_authorizer_id(scope, 'Id', core.Fn.ImportValue('exported-authorizer-id'))`

### Proposed Solution

### Other

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

Start by locating the CognitoUserPoolsAuthorizer entry point and its existing authorizer construction APIs. Check how an authorizer ID could be supplied to support the from_authorizer_id usage described in the issue, then verify that an imported authorizer can be passed to aws_apigateway.Method instead of aws_apigateway.CfnMethod. Done means the cross-stack authorizer ID can be imported and used by Method.

Written by the indexing model from the issue text.