Kubernetes ConfigMap has an upper limit in size on GKE

Currently, the set of trusted roots for ctfe on GKE is stored in a `configmap`, but there's a size limit on this, which could mean that logs trusting a large number of roots might have issues when configuring.

Start by tracing how ctfe's trusted roots are stored in the Kubernetes ConfigMap on GKE; the payload names no files or tests. Determine the applicable ConfigMap size limit and a storage approach that supports large root sets, with completion shown by reliable configuration for logs trusting many roots.

Written by the indexing model from the issue text.