Support listing media by size, upload data, room, user etc in the admin API

This issue has been migrated from [#11986](https://github.com/matrix-org/synapse/issues/11986).

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**Description:**

When trying to reduce the size of your media repo it would be useful to be able to query the admin API to list (the largest) files by user / room etc.

This would let you ask questions like "what rooms are the largest files in", "what rooms are taking the most space", "what user is uploading the most media by size" and then using this output to make delete calls.

Start by reviewing the existing admin API and media repository entry points. Define a listing that can identify media by size, uploader, room, and upload data, and confirm that its output supports subsequent delete calls.

Written by the indexing model from the issue text.