feat: did-plc package

[fforbeck]

Add universal did:plc resolver and signature verifier package

This PR introduces the @storacha/did-plc package, providing universal (Node + browser) utilities for working with the did:plc method.

Initial features include:

• Resolving did:plc identifiers to their DID Documents via the public PLC directory.
• Verifying Ed25519 signatures against all verification methods in a did:plc DID Document (current owner check).

Related: storacha/project-tracking#463

[fforbeck]

@alanshaw this is the first iteration, but still in progress - I'm mapping out the places we need to change to support the did:plc type, and adding any logic that may be required to parse/handle that info.

[fforbeck]

I will break it into 2 PRs:

• 1. Adding the did-plc package
• 2. Updating the existing code to support did:plc type

[alanshaw]

@fforbeck suggestion: collapse the @storacha/did-mailto into a @storacha/did package where I can import @storacha/did/mailto or @storacha/did/plc?

[fforbeck]

@fforbeck suggestion: collapse the @storacha/did-mailto into a @storacha/did package where I can import @storacha/did/mailto or @storacha/did/plc?

I think it's a good idea, but I'm wondering if it's worth the effort. From what I understand, the goal is to eventually get rid of did:mailto, so I would keep them separate initially.