Alongside my attempts to replace $Shape with Partial in the preceding diffs, I'm also looking to replace our custom type Shape.
Doing this for MultimediaUploadExtras revealed a type error. We can't pass dimensions in as MultimediaUploadExtras in native/avatars/avatar-hooks.js on line 71, because dimensions is read-only but MultimediaUploadExtras is not, implying that the call to callUploadMultimedia might mutate it, which would be a type error.
We resolve this by making MultimediaUploadExtras read-only. Also including updates to MultimediaUploadCallbacks, which I made read-only as well as converting to use Partial. (Though this was not necessitated by any type errors.)
Depends on D10085