Add a SetSecretClientOptions to the AzureAppConfigurationKeyVaultOptions #274

I know this was partially covered in #232 but it would be better to provide an extension to supply default SecretClientOptions for the setup. This way we could write:

options.Connect(appConfigUrl, credential)
  .ConfigureKeyVault(vault =>
  {
    vault
      .SetCredential(credential)
      .SetSecretClientOptions(clientOptions);
  });

Using the AzureAppConfigurationKeyVaultOptions.Register(SecretClient) has the drawback of the client code having to know what all the key vault connections are at startup. Rather than allowing that to be masked behind the App Configuration and it just being a matter of the credential having permissions to talk to the various key vaults.