Indexed UCAN store

[expede]

In order to cache UCAN validations, the server needs to perisist some information about UCANs that it's previously validated. This lets us skip validating signatures, and speeds up searching for a UCAN that can act as a proof.

Here is a link to the relevant section in the tech design docs.

Children:

• Persist & index new UCANs to Capabilities store #183
• Short circuit UCAN validation #184
• UCAN Revocations #185

ETA: 2023-10-31