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

New tracking issue for https://github.com/Azure/azure-sdk-for-python/issues/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.