Taproot keysigning is confusing and the API is unhelpful

Right now in our Satisfier API we have a lookup_tap_key_spend_sig method which takes no arguments and returns an Option<Signature>.

The premise, I guess, is that there is only one valid key that can sign a keyspend and the satisfier should know this so there's no need to pass in any extra context.

Well, when developing software you might not know the key, at least initially, because

• The policy compiler will pick a sorta-random key from your policy to be the keyspend key, and it doesn't tell you which one it picks, so you need to know that it does this and then inspect the resulting descriptor
• The key in the policy isn't even the key that you sign with. You need to tweak the key in the policy.

This would be greatly improved by the API providing a Pk and maybe also a TweakedPublicKey and maybe even the taptweak? And the docs should be improved to explain these things.

In general I feel the satisfier API could be greatly improved to provide more context and information to the satisfier.

Related to #742

Start by reading the Satisfier API documentation for lookup_tap_key_spend_sig and the discussion in related issue #742. Clarify which key and tweak context the API should expose, then define the documentation changes needed to explain policy-selected and tweaked keys. Done means the API proposal and documentation scope are agreed.