Add API to test whether a given Galois key is present

When loading stored keys from disk, we don't know which shifts are allowed by the deserialized rotation keys. We should add an API to test whether a given rotation is supported by a set of Galois keys so that higher-level code can verify that keys loaded from disk have the necessary rotation keys.

No files, tests, or entry points are named. Start by locating the APIs for loading stored rotation or Galois keys and inspect how supported shifts are represented; the work is done when callers can query whether a given rotation is supported by a deserialized key set, with coverage for supported and unsupported rotations.

Written by the indexing model from the issue text.