Function to find latest, uninterrumpted provenance attestation

[puerco]

In order to compute the Since date of PROVENANCE_AVAILABLE we need to write a function that finds the oldest commit with a provenance attestation and return its date.

In general, the dates of control sets are reliable as the other controls have their since dates set correctly, meaning we can compute a reliable date. But, for completeness, we should also pack PROVENANCE_AVAILABLE with its own since date.