[native] expose prekey generation to js

Summary: we want to send a contentPrekey and a notificationPrekey to the identity service on registration

Test Plan: console.log(await commCoreModule.generateAndGetPrekey()) returns a prekey string

Reviewers: atul, marcin

Reviewed By: atul, marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9076