Refactor third-party caveat encoder/decoder to caveatid package.

Remove non-public-key third-party caveatid encoding for now (this can always be added
back later as an alternative implementation of bakery.CaveatId{Encoder,Decoder}).
Updated httpbakery & tests accordingly.
I'm not sure that these endpoints deserve to be in the exampleSuite type. We only have one example here, but others might use different services.