feat: add fallback TableProvider to FederatedTableProviderAdaptor

[phillipleblanc]

Adds a fallback TableProvider that can be used in DataFusion contexts without the federation optimizer in-use, which provides a graceful fallback mechanism.

[backkem]

LGTM.
One thing I'd like to explore is if we can disable the fallback if the optimiser is registered. Just to avoid confusion where one (wrongly) assumes the federation is done but in reality the fallback is being used. But this can be picked up in a follow-up.

[phillipleblanc]

LGTM. One thing I'd like to explore is if we can disable the fallback if the optimiser is registered. Just to avoid confusion where one (wrongly) assumes the federation is done but in reality the fallback is being used. But this can be picked up in a follow-up.

This should be possible, since we are passed the SessionState into scan and insert_into - and SessionState has the optimizer rules. Unfortunately there isn't an accessor method to get the currently loaded optimizer rules (there is for physical_optimizers, but not the logical ones). I'm guessing a PR into DataFusion would be accepted to add those methods, and then we could check here if the federation rule is present - and return the error regardless of a fallback table provider.