Custom Domains: Enable adding basepath mappings in Domain property

## Related Issues
#783
## Description
Custom Domains Milestone 3 is to enable multiple domains to multiple Apis mapping. The task tracks adding specific basepath mappings.

SAM input
```yaml
Type: AWS::Serverless::Api
Properties:
Domain:
- DomainName: example.com
Certificate: !Ref MyApiExampleDomainCertificate
BasePathMappings:
- BasePath: somepath # BasePath takes a string, or a list of strings. Required value.
Api: !Ref MyFirstApi # Default: !Ref MyApiAdvancedDomain; use case: mapping this domain to additional APIs
Stage: '' # Default: {MyApi.Stage} Defaults to the current Api's stage
- DomainName: another.com
BasePathMappings:
- BasePath: [anotherpath, andanother]
Api: !Ref MySecondApi # Default: !Ref MyApiAdvancedDomain; use case: mapping this domain to additional APIs
Stage: !Ref MySecondApi.Stage # Default: {MyApi.Stage} Defaults to the current Api's stage
Certificate: !Ref MyApiAnotherDomainCertificate
EndpointConfiguration: REGIONAL # this is default, so same as not having this here
```