Seed YAML files for API definition

[TEF-RicardoSerr]

As stated in the meeting I PR the Seed YAML files for the definition of the API

[eliaspc]

A couple of comments/suggestions from Quobis:

• The /sessions endpoints requests and responses' definitions could use different body payloads specifications and avoid manually noting in the fields' descriptions when that field must be included or not. If using more concrete specifications, backends can enforce payload validation at the endpoint level. This is the case of the POST /sessions endpoint and the answer.sdp field or the resourceUrl field, fields that will only be present in the operation responses and not in requests.

• Response examples for POST /sessions, originatorName and receiverName are not of type string as the specification describes.

• How's deviceId expected to be generated consistently across sessions? This is app-dependant but how are we supposed to trust that the deviceId is not re-used across different devices or reset between app reboots in the same physical device? Does it matter, though?

[TEF-RicardoSerr]

Closing this as comentend in the meeting. Refer to #6 to keep with the discussion