Render captions to m.file, m.image, m.video, and m.audio

[nexy7574]

Description

This PR will introduce rendering file captions.
These were introduced in spec v1.10, and have recently seen wide adoption amongst Element and other clients. It is time that cinny had this too.

Sending captions is out of scope for this PR and will be introduced later.

Fixes partially #1646

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code Image showing two uploads - one with no caption, and one with a caption
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[nexy7574]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

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[ajbura]

LGTM. Thanks for the changes!