[rust-node-addon] generate_nonce client method

Summary:
Depends on D12146

We'll need this method to get a nonce that can be signed and sent as part of the SecondaryDeviceKeysUploadRequest

Test Plan: called generateNonce from keyserver.js and got back a nonce

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12151