[protos][identity] Make keyserver_info response non-optional

Summary:
Resolves ENG-7598.
We agreed that keyserver_info shouldn't be optional.

Checked native_rust_library code and web identity client. They both already treat this as required so nothing to do client-side

Depends on D11471

Test Plan:

• Called the GetKeyserverKeys RPC for user without a keyserver - returned "keyserver not found"
• Registered a keyserver
• Now the RPC responded as before

Reviewers: ashoat, varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11546