[keyserver] Handle invalid CSAT for Identity RPCs
Summary:
Address ENG-6544
Similiar to D13992 but for keyserver
Only authenticated RPC calls are wrapped (from auth.proto). Unauth RPC calls are left unchanged
Test Plan: Malformed token stored in the metadata table. When an auth RPC was called, the value was cleared. Subsequent service calls caused new login attempt and recreated the metadata values
Reviewers: varun, kamil
Reviewed By: kamil
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D14029