ADQL left-joins should be translated to equivalent nested when possible

[gitosaurus]

Feature request

The following ADQL:

SELECT * FROM ppdb.DiaSource ds 
LEFT JOIN ppdb.DiaObject dob ON dob.diaObjectId = ds.diaObjectId
WHERE dob.diaObjectId=24624704620855420

can be done with nothing more than

compute(ppdb.id_search(values={"diaObjectId": 24624704620855420}))

because ppdb already has the joined data nested. While the translation is straightforward, the tricky part is recognizing that it's possible. Nested catalogs served by the TAP server may need to have enough metadata that queries with a LEFT JOIN that accomplish nesting can be simplified as above.

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