AppMesh Gateway support

[JohnPreston]

Untested yet, adding support for AWS AppMesh Gateway:

• https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-appmesh-virtualgateway.html
• https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-appmesh-gatewayroute.html

Unfortunately, the Listerners, Match and actions for HTTP and gRPC are not exactly the same as the ones for the Nodes/Router so had to re-implement these properties on their own. Re-used existing AWSProperty whenever possible.

[JohnPreston]

@markpeek Hello again :)
Going to test this today (by actually deploying it to CFN).
Otherwise seems to render fine

[markpeek]

Thanks!