Log sharding strategy for Fulcio

[haydentherapper]

Description

We need to figure out the log sharding strategy. The simplest approach is to follow what other public CAs do, a new log every year accessible from a unique URL. Let's name shards using the year - ctfe.sigstore.dev/2022, ctfe.sigstore.dev/2023, etc. The current log is accessible at ctfe.sigstore.dev/test, so it'd be good to exercise a sharding before GA.

Each year, the TUF targets will be updated by appending the new log key. Cosign already supports verifying an SCT using a set of public keys. The log URL is not referenced anywhere in code, so only log monitors would need to be made aware of a new log shard URL.