[keyserver][lib][native] Rename userId -> userID

Summary:
This is related to my comments under ENG-6883
metadataEmitter was emitting authMetadata with userID, not userId, so getAuthMetadata would throw errors
Had to change UserLoginResponse to fix this.

Test Plan: flow

Reviewers: varun, tomek

Reviewed By: varun

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11134