[Custom Domains][M3] Make API property Domain accept a list of objects

## Related Issues
#783
## Description
Custom Domains Milestone 3 is to enable multiple domains to multiple Apis mapping. The task tracks adding multiple domains in SAM template.
```yaml
Type: AWS::Serverless::Api
Properties:
Domain:
- DomainName: example.com
Certificate: !Ref MyApiExampleDomainCertificate
- DomainName: another.com
Certificate: !Ref MyApiAnotherDomainCertificate
EndpointConfiguration: REGIONAL # this is default, so same as not having this here
```
Resources - Create a 'AWS::ApiGateway::DomainName' and `AWS::ApiGateway::BasePathMapping` for each domain name:api mapping.