[keyserver][lib][native] Rename RelationshipRequest to LegacyRelationshipRequest

Summary:
ENG-9331 is going to require some changes to the API here, so that we can specify for every individual userID whether we want the keyserver to send robotext to a thin thread (and potentially create a thin thread if one doesn't exist).

Rather than overloading the existing keyserver endpoint, I'm going to introduce a new one. In the near term, we'll keep the old endpoint around to support older clients.

Depends on D13436

Test Plan: Flow

Reviewers: tomek

Reviewed By: tomek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13437