Add multi-user key store provider registration support

[johnnypham]

These changes add a public method to register providers into an instance cache at the command-level, allowing users to maintain their own set of providers in the same application.

The current column encryption key (CEK) cache is global so a new CEK cache and signature verification cache is added at the SqlColumnEncryptionAzureKeyVaultProvider level. Each SqlColumnEncryptionAzureKeyVaultProvider is linked to a single TokenCredential, which serves as the user identity.

public class SqlCommand
{    
       // Added API: registers into command-level key store provider cache
       public void RegisterColumnEncryptionKeyStoreProvidersOnCommand(IDictionary<string, SqlColumnEncryptionKeyStoreProvider> customProviders);
}

[DavoudEshtehari]

Can you assess TTL by adding more tests?

[cheenamalhotra]

Overall LGTM 👍