access-client should use the ucanto validator to select proofs to send with a capability invocation

[travis]

Right now we use a rough heuristic to select which proofs in an access-client Agent's proof store to send along with a particular capability invocation. Roughly, this looks through each of the stored proofs and compares it with the can and with of a passed capability specification. Once it finds a proof that matches the given capability, it stops looking:

https://github.com/web3-storage/w3protocol/blob/main/packages/access-client/src/agent.js#L169

@Gozala pointed out today in our code review that it's possible for a proof store to be "poisoned" with a proof that matches a particular can and with pair but is not actually valid. This would result in a user not being able to perform some action even though they do actually have all the proofs they need.

In the short term, @Gozala suggested we send along all the proofs we have that match a particular capability, rather than just sending the first one we find:

#433 (comment)

In the medium to long term, @Gozala proposed using the ucanto validator to do this filtering directly, which will more accurately identify proofs that can authorize a particular capability execution.