Add BackendCapabilities and a KmsBackend::capabilities() method with a
conservative default so callers can discover which lifecycle operations
the active backend supports instead of probing them. Each backend
declares its real matrix (Vault Transit is the only one advertising
version-retaining rotation). Introduce the typed
KmsError::UnsupportedCapability variant for later use by lifecycle
endpoints.
Refs rustfs/backlog#1571 (part of rustfs/backlog#1562)
overtrue
36b1723cec