Short circuit UCAN validation

[expede]

ETA: 2023-10-31

Use the Capability store to skip validation of UCANs that the server already knows about. This likely requires some integration with rs-ucan (e.g. exposing new traits, etc), but not sure at time of writing.

cc @QuinnWilton thoughts? Here's an example schema of how we'd cache this in a DB.

[matheus23]

We don't do this yet.
So far I'm avoiding DB requests in the auth checking code. The clients have to come with all of the UCANs needed to verify the request.

So essentially I'm postponing this until it becomes an issue.