[lib] Move EthersProvider type to separate file

Summary: In the next diff I need to extend this type. I figured it makes sense to move it to its own file... it was always a little weird defining this in ens-cache.js.

Test Plan: Flow

Reviewers: rohan, atul

Reviewed By: atul

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9524