full support for support for MSC4193 Media Spoilers

[dozro]

Describe the problem

i discovered that, afaik, sable, and cinny, only support MSC4193 partially (missing the option to specify, or display the reason why a media file was spoilered)

https://github.com/88572/matrix-spec-proposals/blob/main/proposals/4193-spoilers-on-media.md

so support for page.codeberg.everypizza.msc4193.spoiler.reason

Describe the solution you'd like

maybe a extra field to specify the reason why you spoiler a file

Alternatives considered

just using the body… which is fine, but we should at least be able to display the spoiler reason

Additional context

No response