EQL: introspection function to classify a value #1313

Applications sometimes need to tell an encrypted value from plaintext, e.g. when an API echoes a redacted field back in an update. The ciphertext's internal prefix is a domain-separation detail, not a public contract, so it shouldn't be used for this.

Add eql_v4.describe(jsonb) (name TBD), returning whether the value is an EQL payload, its domain type, its payload version and its identifier.