Add signals, time_type, geo_type args to pub_covidcast_meta

The endpoint supports these, but the client doesn't, e.g. https://api.delphi.cmu.edu/epidata/covidcast_meta/?signals=chng . See the [server code](https://github.com/cmu-delphi/delphi-epidata/blob/204a7149f5deadfacfb0a9923a6416717e693ff4/src/server/endpoints/covidcast_meta.py#L37-L39) for what's actually handled.