refactor: Asynchronous cog/extension loading.

[elenakrittik]

Summary

Fixes #641

Refactors the codebase by removing loop sharing where possible and using asyncio.get_running_loop() instead. The loop is still shared in places where the code runs on a separate thread like KeepAliveHandler and AudioPlayer. This means that the majority of the library can now assume that there is an event loop already running. Other changes are documented in changelog files.

While the amount of breaking changes may seem horrific, practically most (if not all) use cases are trivial to fix: users will only need to paint a plenty of async/awaits here and there and possibly move any initialization logic from on_ready, bot constructors, etc. into setup_hook. For an example of migrating see test_bot changes, or for a real-world example see these three commits on my Avrae fork.

Thanks to the addition of Client.loop property bots that currently do something like bot.loop.create_task will not be affected by the changes (as long as that access happens after the bot is started, of course).

While i have tried my best to test this in as many cases as possible, it would be appreciated if someone can port their bot and see if there is anything suspicious happening. Notably, i haven't yet tested voice sending features, even though they probably should be fine as-is. Tested it using examples/basic_voice.py, works like a charm now!

I would also appreciate some hints on how/where to document this change, as i believe while it is easy to fix, it is still something that we need to make as many users as possible aware of even outside of the changelog.

As for the "Port #xxxx" commits: this initially started as ports of the according PRs to discord.py, but since the 4th or so commit work was done without "stealing" code from there. Not sure if there's anything legal needed to be done?

Checklist

• If code changes were made, then they have been tested ^(mostly)
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• 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, ...)

[elenakrittik]

Question: is this wording okay?

[elenakrittik]

Question: i'm not sure the suggestion is correct (apart from the typo (get => set), i don't know if set_event_loop will work on-the-fly); might be better to remove the suggestion altogether?