Update Index message in p4runtime.proto to support bytes

[antoninbas]

But I feel like we have opened a can of worms: we also need to address the case of counters / meters, for which the index field should probably be changed to bytes as well. It makes the API less convenient to use IMO, but it makes sense in a way since PSA typically uses bit<W> values to index counters in the datapath.

Originally posted by @antoninbas in #329 (comment)

[smolkaj]

Thanks for opening this for tracking. Agreed on all points.