[protos][identity] Add primary device keys to GetKeyserverKeys response

Summary:
Addresses ENG-7161.

After discussions, we concluded that we need primary device public keys for social proof verification.
We can add it in a non-breaking manner to the existing RPC response.

Implemented function to fetch primary device data and added it to the RPC response.

Test Plan:

1. Registered user on mobile
2. Set that user as a keyserver owner and registered the keyserver
3. Called the RPC - returned keyserver keys, primary device's identity keys and username

Reviewers: will, ashoat, varun

Reviewed By: will, ashoat, varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11471