[lib] Introduce useENSAvatar hook

Summary:
More context in the discussion under ENG-3302. We decided to make this a hook to separate fetching and rendering concerns.

This is mostly modelled after useENSNames, except that we only need to handle one fetch at a time in this hook.

Test Plan: I tested with this patch, which uses an ENS name that points to an NFT in the user's wallet that is an SVG data URI: https://gist.github.com/Ashoat/461523a089b78554ce3a36dcf3115661

Reviewers: ginsu, atul

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7165