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.
Yeah, looking at the code below, I think that would be better.
Then we can lose the PublicKey, PrivateKey and Zeroize methods.