Consider adding optional env variable to disable positive hook messages

[Croydon]

I would like to keep the hooks enabled locally, but the positive non-error messages kinda spam the build log which is annoying.

It would be nice if we can introduce an environment variable which disables positive hook messages, so that depending on the hook level only warnings and/or errors are getting outputted.

[uilianries]

It sounds great. We could add a logging level feature, just like CONAN_LOGGING_LEVEL, but now CONAN_HOOKS_LOGGING_LEVEL.