[identity] GetSessionInitializationInfo RPC

Summary:
clients will call this RPC with a user ID and get in response a map of session initialization info (public keys, social proof, etc.) it needs, keyed by the device's ed25119 signing public key.

adding @ashoat to OK the API design

Test Plan: called this RPC from the keyserver and got back the test data i created in DDB. also tested the failure case with an invalid user ID and got back an exception in JS.

Reviewers: jon, tomek, bartek, ashoat

Reviewed By: ashoat

Subscribers: marcin, atul, ashoat

Differential Revision: https://phab.comm.dev/D6904