Allow keyvault to use the azure_sdk_auth_aad::DefaultCredential for client creation

[gzp79]

As I see keyvault requires to create a cliantId + secret to access the the vault. According to my understanding the other authentication methods ex provided by DefaultCredential should be enough to access the vault.

It'b be great to have access to the keyvault using only the AD DefaultCredential (ex managed identity for prod, cli logins for dev, etc). in a similar way as in c#.