Allow comma-separated list of gages for GET Stations endpoint

For the GET Station endpoint: https://streamstats.usgs.gov/docs/gagestatsservices/#/Stations/GET/Station

It currently allows one gage: https://streamstats.usgs.gov/gagestatsservices/stations/12340000

We would like it to allow multiple gages: https://streamstats.usgs.gov/gagestatsservices/stations/12340000,06326510

This would be helpful for https://github.com/USGS-WiM/StreamStats/issues/1567