[web/native] Add signMessage to olmAPI

Summary:
After rebasing the stack on master now both web and native started using the olm methods to sign messages. Because of that this diff introduces a new method to olmAPI -> signMessage. On native it's just an alias for commCoreModule implementation and on web it proxies to a simple signing function on worker.

Depends on D11373

Test Plan: Called olmApi.signMessage on native. Exported the pickled cryptostore from native and imported it on web. Called olmApi.signMessage on web. Made sure that the outputs matched (for the same input).

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11386