fix(transport/grpc): separate options for creds and certs

[noahdietz]

For gRPC, this separates the logic for credential dial options (associated with the WithoutAuthentication option) from the connection-level encryption dial options (associated with the WithInsecure option OR using DialInsecure). This allows a secure connection to be dialed without credentials.

Previously, using WithoutAuthentication alone (i.e. without WithInsecure or grpc.WithTransportCredentials(credentials.NewTLS(nil)), and while using Dial) would result in attempting to dial a secure connection but without any credentials or TLS config, resulting in an error. For example, the following code returns an error:

c, err := grpc.Dial(context.Background(),
  option.WithEndpoint("storage.googleapis.com:443"),
  option.WithoutAuthentication())
if err != nil {
  // grpc: no transport security set (use grpc.WithTransportCredentials(insecure.NewCredentials())
  // explicitly or set credentials)
}

This is not correct. It makes it very difficult for users to, for example, read a public GCS object without credentials, but using a secure connection.

This may seem like a weird use case, but the HTTP version of dial doesn't control connection-level encryption via the WithoutAuthentication option - connection certs loaded here while credentials are loaded here.

Note: This also refactors the use of grpc.WithInsecure, which is no deprecated, to the preferred grpcinsecure.NewCredentials() option.

[noahdietz]

Update to description: The internal param of dial is determined by use of either Dial or DialInsecure. That said, Dial + WithoutAuthentication but still "secure" is a valid use case and this change makes that possible. users should not need to provide the extra transport level security options to dial a connection without creds.

[noahdietz]

The friction here around WithoutAuthentication but still dialing a secure connection is also mentioned in googleapis/google-cloud-go#4490. This will not fix that feature request, but it does reduce the friction of needing to add extra options to use WithoutAuthentication.