Add new option for limiting query results

[havok2063]

Add a new option to limit the query return results, and return top n number. Related to sdss/zora#71

For more context, the feedback was "The idea would be to allow a person to test out a query without having to wait for a really long time.", which I think is to optionally just return a sample of results. In principle I like this idea. Generally this may be easy enough with peewee's paginate option or limit/offset.

The main long-running queries available right now are on target program/carton. Limit doesn't shorten these queries due to the distinct. See

valis/python/valis/db/queries.py

Line 265 in b496410

def carton_program_search(name: str,

Adding limit takes the query https://api.sdss.org/valis/query/carton-program?name=manual_mwm_tess_ob&name_type=carton&observed=true&release=DR17 from ~5 seconds to ~8 minutes. Switching from a distinct to group by reduces the query time with limit from 8 to 4 minutes. With no limit, it's still 5 seconds. Timings are from direct sql in a psql terminal.