feat: support nullable search fields with postgres transforms

[wschurman]

Why

#468 added nulls support for regular order bys, but explicitly disabled it for search pagination due to it adding quite a lot of complexity. It also fixed a bug where using a nullable field in pagination in general would break the cursor condition and omit rows (any tuple containing NULL would be dropped due to postgres behavior).

But applications still need to be able to search fields that are nullable, they just need to provide a mechanism for dictating how to cast them as non-nullable. This PR provides such a mechanism.

How

Allow the user to specify a function to craft the search field SQLFragment from nullable fields. This is required to be a somewhat awkward shape since the library needs to retain control over the field name due to table aliasing in the cursor.

Test Plan

Run new tests.

[wschurman]

• fix: correct pagination behavior for standard pagination with descending ordering clauses #477
• fix: correct precedence parentheses for SQLFragment joins #476
• feat: add method to get pagination cursor for single entity #475
• fix: correct discriminated union type for forward/backward pagination #474
• feat: support nullable search fields with postgres transforms #470 👈 (View in Graphite)
• fix: Apply same viewer context subclass type mechanism to knexLoader and related methods #472
• main

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff            @@
##             main      #470   +/-   ##
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  Coverage        ?   100.00%           
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  Files           ?       110           
  Lines           ?     16135           
  Branches        ?      1440           
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  Hits            ?     16135           
  Misses          ?         0           
  Partials        ?         0           

Flag	Coverage Δ
integration	23.86% <99.33%> (?)
unittest	95.03% <66.66%> (?)

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[wschurman]

Merge activity

• Feb 27, 2:15 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Feb 27, 2:16 AM UTC: Graphite rebased this pull request as part of a merge.
• Feb 27, 2:19 AM UTC: @wschurman merged this pull request with Graphite.