[native] Modify getUserPublicKey() JSI function to return ClientPublicKeys

Summary: Right now getUserPublicKey() returns stringified JSON, we want an object of type ClientPublicKeys instead.

Test Plan:
Added the following to privacy-preferences.react.js and logged values to ensure that they were as expected:

React.useEffect(() => {
  (async () => {
    await commCoreModule.initializeCryptoAccount('ABC');
    const { curve25519, ed25519 } = await commCoreModule.getUserPublicKey();
    console.log(curve25519);
    console.log(ed25519);
  })();
}, []);

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6115