feat: Allow the same resource scheme for multiple plugins

[matheus23]

Pretty much what it says on the tin.

I moved on to using the same resource for all capabilities in the fission server, in anticipation of how the subject works in UCAN 1.0. (I'm basically using the resource field as the subject now.)

This means the only resource scheme that exists is did:..., but I still have separate abilities like all the account abilities account/read, account/manage, etc. and also capability/find, in a different plugin.

Them being different plugins, but having the same scheme means you need to test all plugins to find the first capability that parses.

This is the change this PR makes.

[QuinnWilton]

This makes sense, thanks! Do you think it should also be possible for multiple plugins to succeed?

[matheus23]

Hmm. Good question 🤔

So far, I haven't needed that.
It can be weird to see that ucan.capabilities() is an interator with more items than in the original UCAN, plus them encoding to the same data. That'd make me go "is that a bug?".
Perhaps if the API were different. Something like find_capabilities and you provide it with the resource, ability and caveat types you expect, and then it filters out the relevant ones. Then it'd be totally fine to have multiple plugins succeed (even desired), although effectively at that point you could also just provide the plugin you're expecting.