[Protos] Use outbound key bundle for keyserver information

Summary:
Keyserver makes use of both content and notif keys. Unify the values
to match in both cases.

The keyserver endpoint is going to remain, since it differs from
GetOutboundKeysForUser by only selecting the keyserver.

Part of https://linear.app/comm/issue/ENG-3941

Test Plan: N/A, the endpoint hasn't been implemented. This is more about design

Reviewers: ashoat, bartek, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8090