feat: server validation hooks

[Gozala]

This pull request partially implements capability validation on the server. Here is rough overview of the changes:

1. Server.create now requires id: Identity field and will check validate that received invocations have it as audience.
2. Server.create now takes optional canIssue, my, resolve and authority fields and passes them via context argument so that they can be passed to validator's access function. If not provided sensible defaults are derived.
3. New handler.js module on the server exports provide(capability, handler) function which can be used to bind capability parser with it's handler. This could be used to simplify UCAN validation until better API is added. (see handler.spec.js for the examples)
4. ⚠️ URI parser thingy got replaced with slightly different URI thingy. Now you can use with: URI when you don't care about protocol and you have to use URI.match({ protocol: 'did:' }) when you want URIs with a specific protocol.
   • API changes was required to make some typings work better as TS would sometimes fail with the older one.
5. Link decoder had been added, which is roughly equivalent of URI decoder but for CIDs. (I found myself writing it all over the place and it made sense to just have a thing out of the box).
6. I end up reexporting dag-ucan from @ucanto/core to avoid depending on both from everywhere. This way less things can get out of sync.