feat: Add Generic Principal Decoding

[Goddhi]

generic decoding functions for principals (signers and verifiers) that can automatically detect and decode the correct implementation based on multiformat codec prefixes.
issue

[volmedo]

hey @Goddhi, thanks for the PR and sorry it's taking a while for us to review. I'll review and provide feedback shortly. In the meantime, take a look at the issue in the test run 🙏🏻

[Goddhi]

@volmedo thanks for taking a look! I’ve fixed the issue that caused the CI failure. 🙏

[alanshaw]

@Peeja did you need this feature - would you care to review?

[Peeja]

@Goddhi Can you point me to what this was for? I think I'm missing context, I'm not sure why this was needed.

[Goddhi]

@Peeja Thie PR addresses the issue described in this issue 46
The problem is the principal.Signer and principal.Verifier can be encoded polymorphically using Encode(), which returns multiformat bytes containing the algorithm identifier. However, there's no way to decode those bytes back to the correct implementation automatically. Users must know in advance whether they're decoding an Ed25519 or RSA principal.
from the PR i made i introduced DecodeSigner(encoded []byte) this automatically decodes to the correct signer implementation based on the multiformat code and
DecodeVerifier(encoded []byte) automatically decodes to the correct verifier implementation.

[Peeja]

@Goddhi Ah! Of course, sorry, I'm not sure why I missed the issue link before.

[Peeja]

This looks great! Let's just move it around a bit. Let's move decode.Signer() to "github.com/storacha/go-ucanto/principal/ed25519/signer".Decode(), and decode.Verifier() to "github.com/storacha/go-ucanto/principal/ed25519/verifier".Decode().

As for decode.Principal(), I'm not sure we really need that one. I think if you don't know which you have, something weird is going on, and if you really need to, it's not hard to do this handling and switching yourself. Let's drop that one.

[Goddhi]

Thanks for the clarification, @Peeja That makes sense, keeping the decode logic with the specific implementations is cleaner.
i just made a new commit based on your suggestion, I removed the decode.Principal() entirely.