Proposal: Define `MultihashVerifier` interface

Goal

As per https://github.com/ipld/js-car/issues/123 we do need a solution for verifying hashes in CAR files without shipping all the MultihashHasher-s with the CAR library.

Proposal

Define a separate MultihashVerifier interface that can be implemented by all of our hashers. It could look like this:

export interface MultihashVerifier {
  // Fails if source produces a different multihash
  verify(mustihash: Uint8Array, source: Uint8Array): Await<{}>
}

You may be thinking can't I just use MultihashHasher to create a digest from the source and then compare. While you could there are some subtle cases where having MultihashVerifier would be a better option:

1. Hashing algorithms like blake3 provide incremental verifiability, allowing a more optimal implementation strategy.
2. It is possible to implement e.g. trust: (config) => MultihashVerifier that could be leveraged as an explicit opt-out as opposed to subtle option that is easy to miss.

Start by reviewing the linked js-car issue and the existing MultihashHasher interfaces and implementations in this repository. Determine the verifier contract, including failure behavior and support for incremental verification, then define the interface and establish how hashers can implement it. Done means the proposal is resolved in a consistent, documented API.