[native] Add Olm account sign function to commCoreModule

Summary: This exposes the function to sign messages with Olm account to JS. This is a counterpart of Account.sign() on web/keyserver.

Test Plan:
Used online Ed25519 tool to verify generated values:

await commCoreModule.initializeCryptoAccount();
// used primaryIdentityPublicKeys.ed25519
const keys = await commCoreModule.getUserPublicKey();
const message = JSON.stringify({ hello: 'world '});
const signature = await commCoreModule.signMessage(message);
console.log({ keys, message, signature });

Reviewers: marcin, kamil

Reviewed By: marcin, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11037