New serverless pattern - Private Amazon Api Gateway Custom Domain Name

[usama-khan98]

New SAM templates to create Private REST API Gateway with Private Custom Domain Name.

The SAM template deploys a Private Amazon API Gateway with a Private Custom Domain Name mapped to deployed stage. This template also create a Route53 CNAME record in a Private Hosted Zone to map the Private Custom Domain Name (e.g. private.mydomain.com) to the Target VPC Endpoint DNS Name. (e.g. vpce-abcdefgh123456789-abcd1234.execute-api.us-east-1.vpce.amazonaws.com).

GitHub PR for template: #2523