Provide GET method | webrtc-registration

[stroncoso]

Problem description
There's no way to retrieve active registrations from the API consumer side (app). Only new registers or updates are feasible. Detected on #129 an API consumer can not confirm what subscription is active, and neither update it once reconnected from network reasons.

This leads to bad resources management on the network side and a limitation on the common usage of the API endpoint by the apps and developers.

Possible evolution
Include a GET method
Under valid request, a GET operation should bring the following responses:

200 OK + registrationStatus
404 Not found. Meaning unregistered.

The registration response:

• if only the deviceId is included, it will bring all registrations for that deviceId
• if the registrationId is included along the deviceId a single object will be retrieved.

It is not possible to retrieve registrations from other devices.

Alternative solution
not discussed

Additional context
Check previous discussion context at #129