Merge slsa-policy repo with this repo

This repo https://github.com/slsa-framework/slsa-policy

slsa-verifier would then become the single source for:
- APIs to verify attestations (the current scope of the repo so far)
- APIs to generate attestations (VSA, SLSA VSA, publish attestation, deployment attestation) - currently in the slsa-policy PoC repo
- APIs to run policies - currently in the slsa-policy PoC repo