* we should look for both embedded relationship and relationship key,
so in cases like for commit, when there's a full commit data on
"commit" property, and only id at "commit_id", we will use commit data
* we can't add @type to V2 fallback, because in other places we chack
for @type to distinguish V2 and V3 payloads
* there's no need to include a record in "included" if there's only a
type and an id there
This commit adds adapters and serializers for v3, but also a fallback
serializer for v2, which allows to handle v2 and v3 payloads at the same
time. This is needed, because when we use v3 endpoint for one of the
models (in this case repo), we can also get embedded records of other
types (like branch or build).
