In #23977 we moved the keyring into Raft, which can expose key material in Raft
snapshots when using the less-secure AEAD keyring instead of KMS. This changeset
adds tools for redacting this material from snapshots:
* The `operator snapshot state` command gains the ability to display key
metadata (only), which respects the `-filter` option.
* The `operator snapshot save` command gains a `-redact` option that removes key
material from the snapshot after it's downloaded.
* A new `operator snapshot redact` command allows removing key material from an
existing snapshot.
