Warn about messages which are not on the HYBRID API level

[stapelberg]

(This is related to issue #10)

The open2opaque tool expects .proto files to be on the HYBRID API level so that the new accessor methods are already available, but the current code still builds (see https://protobuf.dev/reference/go/opaque-migration/#setup).

We should consider making the tool print a warning when it is invoked for proto messages which are not on the HYBRID API. Because open2opaque processes Go source code (not .proto files), we need to detect the API level based on the protogen struct tag in the Go message type.