[Feat]: Define attestor types that leverage the intelligence gathered by other attestors

[ChaosInTheCRD]

Describe the solution you'd like:

Currently we have a variety of attestor types:

• PreMaterial
• Material
• Execute
• Product
• PostProduct

Each attestor type runs at a different point of a witness run invocation, and therefore allows a user to execute the attestor at the appropriate place given its function.

There is an emerging need for some construct in the codebase that represents attestation generation that depends on the metadata generated by other attestors (e.g., Link attestor and SLSA Provenance). Naturally, these attestors need to run at the end, or at least after the attestors that they depend on.

One suggestion that I think I have heard from others is the idea that the existing attestor types are redefined as "collectors" and the attestor types I am referring to in this issue are given their own construct. They can then use the collectors to build up their attestations.

User value:

Witness being able to generate attestations for predicates defined elsewhere in the community (e.g., SLSA Provenance) by simply leveraging the attestors that already exist.