Add Valley Metro (covering the Phoenix metro area)

[nogweii]

Agency name: Valley Metro City/region/other locality: Maricopa County, AZ (Phoenix, Tempe, Chandler, and more.) Link to alerts feed docs/info (if known): GTFS-RT URL: https://app.mecatran.com/utw/ws/gtfsfeed/alerts/valleymetro?apiKey=4f22263f69671d7f49726c3011333e527368211f&asJson=true More documentation: https://www.phoenixopendata.com/dataset/general-transit-feed-specification (linked from Valley Metro's website: https://www.valleymetro.org/contact/developers-resources)

Other comments/discussion: Valley Metro is a really generic name. I wonder if you want to use that or some locality-prefixed name like 'phxvalley` 😆 .