Add runtime group listing function (#627)

apigrpc/apigrpc.pb.go

apigrpc/apigrpc.swagger.json

console/console.pb.go

go.mod

go.sum

server/api_group.go

server/core_group.go

server/runtime_go_nakama.go

server/runtime_javascript_nakama.go

server/runtime_lua_nakama.go

vendor/github.com/heroiclabs/nakama-common/api/api.pb.go

vendor/github.com/heroiclabs/nakama-common/api/api.proto

.../github.com/heroiclabs/nakama-common/rtapi/realtime.pb.go

...or/github.com/heroiclabs/nakama-common/runtime/runtime.go

vendor/modules.txt