Add support for Irsa query for ZTF lightcurves

The existing `Irsa` module can query the [public ZTF catalog](https://www.ztf.caltech.edu/page/dr1):

```
from astroquery.irsa import Irsa
from astropy.coordinates import SkyCoord
sc = SkyCoord('01:53:49.67 +20:20:47.7',unit=(u.hourangle,u.deg))
Irsa.query_region(coordinates=sc,catalog='ztf_objects_dr1', radius=1.5*u.arcsec)
```

However, the returned result is a short list of matching objects, and there is no way to retrieve the underlying photometry. (This is in contrast to the behavior when issuing the same query with `catalog='ptf_lightcurves'`).

There is an [API](https://irsa.ipac.caltech.edu/docs/program_interface/ztf_lightcurve_api.html) for the lightcurve service which could be used to enable direct queries for ZTF lightcurves.