REST API: Allow rest_get_avatar_urls() to accept full user, post, or comment objects, rather than just an email address, to provide better flexibility for alternative avatar data.
Since the function uses `get_avatar_url()` internally, which already supports it, this should not have any backward compatibility concerns.
Props donmhico, dshanske, pputzer, joehoyle, TimothyBlynJacobs.
Fixes #40030.
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