Update names for more clarity #231

Problem description
Changes in several resource and schema names would make the APIs more easy to understand.

Possible evolution
Suggest to rename names as follows (impacts yaml and test files)

in webrtc-call-handling.yaml

• MediaSessionStatusChange -> MediaSessionUpdate - the current name reads as an implicit notification while it is a PUT operation
• the API could be simplified by just using session instead of mediasession (see also update to registration yaml below, as with the change to registration as a resource, you no longer need to distinguish the 2 types of sessions.

in webrtc-registration.yaml

• /sessions -> /registrations - this also would enforce the standard pattern /registrations/{registrationId}
• then change RegSession* to Registration* everywhere

Additional context
This is not blocking the current release, but would help understanding of the API by the API Consumers