Feat: MSC2530 support for Media Captions

[cyrneko]

Describe the problem

Cinny currently does not support rendering or sending image captions as per MSC2530. This means:

• Images sent with a caption will not have it visible until hovered to reveal it as Alt-Text
• it isn't possible to send media with a caption
  • Cinny will instead upload the media early, then send both the caption and media in quick succession to emulate media captions.

Describe the solution you'd like

It would be great if support for MSC2530 could be introduced. This would allow for the above issues to be rectified.

Alternatives considered

The alternative is to constantly look in the title of an image preview, or hover over an image to get Alt-Text (which is the image caption). This is not ideal as most users would not notice the image caption in these places, and cannot even read it fully in the image viewer's header.

Additional context

No response