Handle ADQL COUNT(DISTINCT) as a request for unique values

Feature request

The following AQDL:

SELECT COUNT(DISTINCT diaObjectId) FROM ppdb.DiaObject

can be translated to:

import numpy as np
# This is just loading data from the diaObjectId column.
np.unique(compute(ppdb["diaObjectId"]))

# It may also be possible to do it via Dask:
compute(ppdb["diaObjectId"].unique())

There is quite a lot to translate here, since the COUNT and DISTINCT are not being done as part of compute, but outside of it. Despite the long reach, however, the concept here is a common one, and detecting that the user is asking for it and turning it into the most efficient LSDB version is still worth doing.

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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.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.