Reuse Fulcio cert

[imjasonh]

Today we request a new Fulcio cert for each thing we sign or attest, which can become a lot.

Instead we should cache a Fulcio cert for some duration (5 minutes) and reuse it for all signings done during that time. The details of the cert (identity, GH workflow ID, etc.) will be identical for all the certs, it'll just mean a lot less traffic and latency talking to Fulcio.

[jonjohnsonjr]

Started on something for attest: https://github.com/jonjohnsonjr/secant