[keyserver] add new compareUsers function to rust-node-addon

Summary:
adding a new function that just calls the compareUsers RPC and returns a userComparisonResult, containing two arrays: usersMissingFromKeyserver and usersMissingFromIdentity

Depends on D6984

Test Plan: this gets tested in the next diff

Reviewers: ashoat, jon

Reviewed By: ashoat, jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6997