signing: Use all logs listed in signingconfig

[jku]

SigningConfig can contain multiple rekor instances (from different operators): I understand the intent is that clients can create entries in each one.

Currently we only contact one log but could handle multiple. This is not an urgent improvement since there is no plan to have multliple logs (from different operators) in the public good instance signingconfigs.

[jku]

this is followup from #1407