Add methods for getting app command IDs/mentions from cache

[Flame442]

Description of the changes

Adds bot.get_app_command_id and bot.get_app_command_mention which pull from the ID cache built when RedTree.syncing.

Potential future work:

• Hook into CommandTree.fetch_commands to update the cache manually using the fetched command data
• Migrate the current data structure to allow storing guild specific app command IDs on guild sync, and allow these new methods to be used to pull from that cache as well

Fixes #5976

Have the changes in this PR been tested?

Yes

[Jackenmen]

Potential future work:

• Migrate the current data structure to allow caching on guild sync, and allow these new methods to be used to pull from that cache as well

Having such a cache now would mean we don't need to make these methods asynchronous and such a change to method signature is not really something we can do once the API is released. Is there any reason we wouldn't want those methods to be synchronous?

[Flame442]

I don't mean a memory cache, I mean storing in config guild specific command ids. Currently, only global app command ids are stored, since that is all the [p]slash system cares about.
(Edited original to be more clear)

[Jackenmen]

Oh yeah, I missed that this remark is about guild stuff. Either way, I am still wondering about the in-memory cache thing to get a synchronous API, it does seem a bit unfortunate that it requires awaiting right now.