Allow setting connection options from command line in QuicToyClient This CL also adds a convenience method that can parse a comma-separated list of QuicTags, with corresponding test. The additions in the shared code are only used in QuicToyClient, so this CL does not impact existing behavior in prod. Test-tool-only change PiperOrigin-RevId: 317685170 Change-Id: I83e7737a3e4e40446e6051bd3ec07dfbf81559e2