Translate nearest-neighbor searches from AQDL

Feature request

A query like this:

SELECT 
  o1.diaObjectId AS id1,
  o2.diaObjectId AS id2,
  DISTANCE(POINT('ICRS', o1.ra, o1.dec), POINT('ICRS', o2.ra, o2.dec)) AS d
FROM
  ppdb.DiaObject AS o1
JOIN
  ppdb.DiaObject AS o2 ON o1.diaObjectId <> o2.diaObjectId AND o1.validityStart = o2.validityStart
WHERE
  CONTAINS(POINT('ICRS', o1.ra, o1.dec), CIRCLE('ICRS', 186.84, 7.01, 0.05)) = 1
AND 
  DISTANCE(POINT('ICRS', o1.ra, o1.dec), POINT('ICRS', o2.ra, o2.dec)) < 0.02;

should be translated to:

from lsdb.core.crossmatch.bounded_kdtree_match import BoundedKdTreeCrossmatch
cone = ppdb.cone_search(ra=186.84, dec=7.01, radius_arcsec=0.05*3600)
xmatch = cone.crossmatch(ppdb, algorithm=BoundedKdTreeCrossmatch, min_radius_arcsec=1e-5, radius_arcsec=0.02*3600, suffixes=("_1", "_2"))
xmatch = xmatch[xmatch["validityStart_1"] == xmatch["validityStart_2"]]
compute(xmatch[["diaObjectId_1","diaObjectId_2","_dist_arcsec"]])

This is a difficult one, but it's possible, if the translator knows:

1. Why o1 and o2 should be joined on ID in order to be equivalent to ppdb.
2. That the DISTANCE constraint should be converted into radius_arcsec= (this should be done for existing crossmatch translation anyway, and should be its own issue).
3. To do the validity-start match after the crossmatch. The way the ADQL is written, it would be reasonable to do that search first, but of course it's more efficient to do that post-crossmatch.

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• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
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