Implement basic support for GUILD_MEDIA channels.

[AnotherZane]

Description

Start implementation of GUILD_MEDIA channels (16).
Ref: New GUILD_MEDIA channel type, Media Channels

Checklist

• I discussed this PR with the maintainer(s) prior to opening it.
• I read the contributing guidelines.
• I tested the changes in this PR.

[AnotherZane]

Need opinions on renaming IForumChannel. I considered using IThreadOnlyChannel but that would lead to awkward naming for some classes such as CreateForumThreadJsonRestRequestContent.

Also need to consider if IForumChannel should still exist, however as a new entity (inheriting its renamed version) since the default_forum_layout property currently exists on media channels (in the API) but is only used in forums channels (as per documentation).

Perhaps we refactor to IMediaChannel which can be inherited by IForumChannel?