Add Support for EndpointConfiguration IpAddressType

[iph]

Hello!

API Gateway recently released IpAddressType in many of their resources. E.g. https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-apigateway-domainname-endpointconfiguration.html#cfn-apigateway-domainname-endpointconfiguration-ipaddresstype

If folks want IPv6 and are on SAM, we probably need to plumb this property through anywhere EndpointConfiguration is used.

Based on the guidelines, the proposed solution would be to passthrough the name IpAddressType where EndpointConfiguration is used.

May try to hack my way through this :)

[iph]

Proposed impl (need to actually see if it works now :) ) develop...iph:serverless-application-model:develop