[commtest] Add helpers for ED25519 signing messages

Summary:
Added some utilities to Commtest that imitate OlmAccount.sign() which is used for nonce challenge in our RPCs

All dependencies already used elsewhere

Test Plan: Commtest compiles, other tests pass. Logic tested in the next diff

Reviewers: varun, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11467