Add Private API Gateway Option

[KevinHock]

Is your feature request related to a problem? Please describe.
I would like to be able to make the API Gateway Private and then associate a VPC Endpoint with it.
This would enable say, me putting all the resources in their own account, and the only thing being exposed from that account being a VPC endpoint.

https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-private-apis.html#associate-private-api-with-vpc-endpoint

This is somewhat related to # FIXME (derek.wang) Switch to PRIVATE later and attach some VPCs at

rudolph/deployments/terraform_modules/santa_api/main.tf

Lines 11 to 13 in fcb3c40

	endpoint_configuration {
	types = ["REGIONAL"] # FIXME (derek.wang) Switch to PRIVATE later and attach some VPCs
	}