Slighty refactoring for the cross api spec

An API package has been created in common to store the API base class,
request and response classes, API validation and schemas.

The processing of the requests and responses will be added in a follow
up patch.

Change-Id: I3068b6fffde2d4d88a6009ca2ed703b38b791273
Partial-Implements: blueprint cross-transport-api-spec
Merge "Slighty refactoring for the cross api spec"