Add unstable /keys/claim endpoint which always returns fallback keys. (#15462)

It can be useful to always return the fallback key when attempting to
claim keys. This adds an unstable endpoint for `/keys/claim` which
always returns fallback keys in addition to one-time-keys.

The fallback key(s) are not marked as "used" unless there are no
corresponding OTKs.

This is currently defined in MSC3983 (although likely to be split out
to a separate MSC). The endpoint shape may change or be requested
differently (i.e. a keyword parameter on the current endpoint), but the
core logic should be reasonable.