voltha-protos: voltctl command to show ONU OMCI counter statistics

For the purpose of debugging a new voltctl command should be introduced to display the following counter values of OMCI communication between openonu-go-adapter and ONUs:

Number of TX/RX-frames (separated for baseline and extended format):
TX AR=1, RX AK=1, TX AR=0, RX AK=0 (AR: Acknowledge Request, AK: Acknowledgement)
Number of repeated TX-frames due to missing answer
Number of TX-frames running into timeout (even after repetition)

Additional documentation:
https://docs.google.com/document/d/1-n0LzHYxp4VK_FhgD6FkU8raKdWpQ7uWLXEiqwHvU3c/edit#