[WIP] Support dynamically updating authentication headers

[faph]

[Work in progress]

Initial basic refactor of BasicAuth object, generalized as an async callable/coroutine.

What do these changes do?

TODO

Are there changes in behavior for the user?

TODO

Related issue number

#6915

Checklist

• I think the code is well written
• Unit tests for the changes exist
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[webknjaz]

__call__() is for normal callables, it'd be weird for it to be async. There's an __await__() that exists for awaitables. Maybe don't go for magic methods? It looks like you're reinventing a protocol pattern.

Also, it feels like for the use-case you described using an async generator interface could be a better fit then an async function.

[faph]

Thanks for the feedback @webknjaz.

It might be useful to collect these high-level design considerations in the corresponding issue #6915. I was using this PR largely just to give an example what an implementation could look like.