Add a new base class for loop scoped variables in asyncio

[kumaraditya303]

We have several loop-scoped global variables now. Some are Python objects, but llist_node is a plain C structure.
Let's make a super-base C Extension class where we can put all these objects (both python and plain C) and inherit BaseEventLoop from it.
I personally feel that we will have more loop-scoped things like llist_node in the future; now is a good time to settle a place for them.

Originally posted by @asvetlov in #128416 (comment)

[graingert]

Could you expose these so user code can use it? I'd like to implement RunVars - which are like ContextVars but for a particular asyncio.run. currently we use WeakKeyDictionaries from loop to object

We also use WeakKeyDictionaries to map from task to the CancelScope it's in. I'd like to make one WeakKeyDictionary per event loop rather than having one global one