Implement method to sign message

Summary: This differential implements method to sign a message using private ed25519 key of an olm account. Counterpart method to verify signature is implemented as well

Test Plan: In CommCoreModule call this method on cryptoModule attribute on some data. Copy returned signature. Pass this signatur back to verify method, ensure app does not crash.

Reviewers: tomek, atul, bartek, kamil

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6821