[web] Introduce placeholder WebEditUserAvatarProvider

Summary:
Very very naive implementation of WebEditUserAvatarProvider w/ platform-specific props set to dummy values for the timebeing.

This unblocks (non-image) avatar work (remove avatar, emoji avatar, etc) and lets us defer the image avatar stuff for later. Will obviously have to come back and implement these when it comes time to work on image avatar work, just want to start w/ the simplest flows first.

Test Plan:
Made sure I could "get values out of the context" from EditUserAvatar w/ some logging:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8249