[Feature] Media channel support (#2725)

* initial commit

* oops

* another typo -_-

* Update AttachmentFlags.cs

Made this on my phone lol

* Update AttachmentFlags.cs

* -line

* initial impl

* some guild methods for media (and forum) channels

* file attachment can be a thumbnail

* can't edit media channel layout

* updatess

* Update ChannelPermissions.cs

* typo

src/Discord.Net.Rest/Entities/Channels/RestGuildChannel.cs

@@ -42,6 +42,7 @@ namespace Discord.Rest
                 ChannelType.Text => RestTextChannel.Create(discord, guild, model),
                 ChannelType.Voice => RestVoiceChannel.Create(discord, guild, model),
                 ChannelType.Stage => RestStageChannel.Create(discord, guild, model),
+                ChannelType.Media => RestMediaChannel.Create(discord, guild, model),
                 ChannelType.Forum => RestForumChannel.Create(discord, guild, model),
                 ChannelType.Category => RestCategoryChannel.Create(discord, guild, model),
                 ChannelType.PublicThread or ChannelType.PrivateThread or ChannelType.NewsThread => RestThreadChannel.Create(discord, guild, model),