[keyserver][lib] Introduce update_relationships2 keyserver endpoint

Summary:
We'll update the client code to use this new keyserver endpoint in the next diff. I did this refactor as part of my work on ENG-9331.

Depends on D13439

Test Plan: I tested this new endpoint as part of my testing of ENG-9269

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13440