Add more attributes to AppCommandChannel

[Sacul0457]

Summary

This PR (once again) adds more attributes to AppCommandChannel

Rational behind some of the decisions made:

• Return None if the channel doesn't have a certain field. This avoids implying a channel has the feature/field when it actually doesn't. For instance, slowmode_delay with CategoryChannel, if we return 0, it implies that it has a slowmode of 0s, when it actually doesn't have that feature.
  • Breaking Change: With that said, this also means slowmode_delay is now Optional[int] which makes it a breaking change. If this isn't a good idea, i'll revert it!

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Sacul0457]

Draft until there's more updates on discord/discord-api-docs#7957