[Key Vault] Ensure common API between KeyVaultCertificate and CertificateOperation to distinguish them

[laiapat]

New tracking issue for #28242.

To avoid breaking stable APIs, this solution may need to involve adding a new, shared property to the two classes. Tests will need to be written to validate that any existing strategies to differentiate return types -- e.g. a try/except using AttributeError -- continue to work with the new implementation.