[4.5] Expose AsyncRefresh capabilities

[halcy]

4.5 adds a new AsyncRefresh header and endpoint. We need to expose this somehow.

First draft plan:

1. Base entity type gets AsyncRefresh info. We add a function to retrieve this info. This is the most baseline support possible.
2. Add the asyncrefresh endpoint. I think this is best implemented barebones-ish: Pass in an asyncrefresh ID, get out a non-typed dict, pass in a typed dict, get out a typed dict with the asyncrefresh results, throws if not set.
3. Add an additional function that does the asyncrefresh fetch if present, no-op if not (typed dicts only!), respects the provided backoff time, tries again if there's another asyncrefresh header, until all the refreshing is done. This is the default behaviour, can be turned of by arguments as needed.

Comments welcome.