[native] Allow rainbow scheme in iOS app

Summary:
This lets us open rainbow:// scheme URLs, which open in the Rainbow Ethereum wallet app.

We should do this for other wallet apps too; ENG-2542 tracks this.

Depends on D6036

Test Plan: Confirm Comm app can open Rainbow app during SIWE flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6037