GOES NRT data from goes_xrs

### Describe the feature

Following discussion with @alasdairwilson it would be nice to have the goes_xrs submodule be able to download the GEOS NRT json file and return a timeseries

something along the lines of

```python
from sunkit_instruments import goes_xrs
goes_nrt = goes_xrs.get_nrt()
``` 

