[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.Interactions/TypeConverters/SlashCommands/DefaultEntityTypeConverter.cs

@@ -69,6 +69,12 @@ namespace Discord.Interactions
                 _ when typeof(ITextChannel).IsAssignableFrom(type)
                     => new List<ChannelType> { ChannelType.Text },
 
+                _ when typeof(IMediaChannel).IsAssignableFrom(type)
+                    => new List<ChannelType> { ChannelType.Media },
+
+                _ when typeof(IForumChannel).IsAssignableFrom(type)
+                    => new List<ChannelType> { ChannelType.Forum },
+
                 _ => null
             };
         }