[Feature] Age restricted (NSFW) application commands support (#2531)

* add `nsfw` to data model & internal methods; add missing property

* add `nsfw` prop to command builders

* add `NsfwCommandAttribute` to Interaction Framework

* working state

* docs?

docs/guides/int_framework/permissions.md

@@ -55,5 +55,10 @@ The amount of nesting you can do is realistically endless.
 > If the nested class is marked with `Group`, as required for setting up subcommands, this example will not work.
 > As mentioned before, subcommands cannot have seperate permissions from the top level command.
 
+### NSFW Commands
+Commands can be limited to only age restricted channels and DMs:
+
+[!code-csharp[Nsfw-Permissions](samples/permissions/nsfw-permissions.cs)]
+
 [permissions]: xref:Discord.GuildPermission
 

docs/guides/int_framework/samples/permissions/nsfw-permissions.cs

@@ -0,0 +1,6 @@
+[NsfwCommand(true)]
+[SlashCommand("beautiful-code", "Get an image of perfect code")]
+public async Task BeautifulCodeAsync(...)
+{
+    ...
+}