Add SecretProperties.managed property

[chlowell]

A secret has a "managed" attribute indicating whether it's managed by Key Vault, i.e. associated with a certificate. SecretProperties expects this attribute but doesn't expose it:

azure-sdk-for-python/sdk/keyvault/azure-keyvault-secrets/azure/keyvault/secrets/_models.py

Line 29 in 65e2f5d

self._managed = kwargs.get("managed", None)

[laiapat]

Resolved by #22368