[Custom Domains][M1] Implement multiple basepath mappings for single domain single api.

[praneetap]

Related Issues

#783

Description

This is a proposal to add Basepath mappings for Custom Domains usecase.

  Type: AWS::Serverless::Api
  Properties:
    Domain:
      DomainName: example.com
      Certificate: !Ref CertificateARN 
      BasePath: [pathone, pathtwo] # '/' is no longer a valid basepath if you have multiple. They all map to Api.Stage.

SAM must create a AWS::ApiGateway::BasePathMapping resource per base path.
SAM must also add the basepath to the swagger.