Adds interfaces for handling Magic Transit - Static Routes

[cehrig]

This PR implements methods for handling Magic Transit Static Routes

• List Routes (https://api.cloudflare.com/#magic-transit-static-routes-list-routes)
• Route Details / Get Route (https://api.cloudflare.com/#magic-transit-static-routes-route-details)
• Create Route (https://api.cloudflare.com/#magic-transit-static-routes-create-routes)
• Update Route (https://api.cloudflare.com/#magic-transit-static-routes-update-route)
• Delete Route (https://api.cloudflare.com/#magic-transit-static-routes-delete-route)

$ go test $(go list ./...) -v -run MagicTransitStaticRoute
=== RUN   TestListMagicTransitStaticRoutes
--- PASS: TestListMagicTransitStaticRoutes (0.00s)
=== RUN   TestGetMagicTransitStaticRoute
--- PASS: TestGetMagicTransitStaticRoute (0.00s)
=== RUN   TestCreateMagicTransitStaticRoutes
--- PASS: TestCreateMagicTransitStaticRoutes (0.00s)
=== RUN   TestUpdateMagicTransitStaticRoute
--- PASS: TestUpdateMagicTransitStaticRoute (0.00s)
=== RUN   TestDeleteMagicTransitStaticRoute
--- PASS: TestDeleteMagicTransitStaticRoute (0.00s)
PASS

[jacobbednarz]

thanking you @cehrig 🍰