Implement MSC4133 to support custom profile fields. (#17488)

Implementation of
[MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133)
to support custom profile fields. It is behind an experimental flag and
includes tests.


### Pull Request Checklist

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* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
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The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
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* [x] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters)

)

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>