Document and enforce compatibility between an aggregate's accumulators

[masonh22]

Which issue does this PR close?

• Closes Document and enforce compatibility of the intermediate state produced by an AggregateUDFImpl's Accumulator and GroupsAccumulator #25660.

Rationale for this change

See #25660.

What changes are included in this PR?

This change adds:

• datafusion_functions_aggregate::testing::check_state_compatibility(): A generic test that identifies cases where the Accumulator and GroupsAccumulator of an AggregateUDF differ. This is exposed as a public function that can be used to test this requirement on AggregateUDFImpls implemented by downstream consumers. The test is run on all built-in default aggregation functions.
• Fixes for approx_distinct() and avg(), which both violated this requirement
  • This takes the changes from Allow the HLLAccumulator used by approx_distinct() to consume state produced by HllGroupsAccumulator #25659 to fix approx_distinct()
  • The Accumulator for avg() would return 0 for the count if it hasn't seen any non-null values. The GroupsAccumulator would return null instead. This change updates the Accumulators to also return null for the count in this case.
• Documentation explaining this requirement and how to check for it using the provided test function.

What is the testing strategy for this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.29878% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.70%. Comparing base (8248a57) to head (59fe8c5).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...on/functions-aggregate/src/testing/state_compat.rs	85.18%	57 Missing and 31 partials ⚠️
...afusion/functions-aggregate/src/approx_distinct.rs	77.77%	7 Missing and 3 partials ⚠️
datafusion/functions-aggregate/src/average.rs	70.58%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25710      +/-   ##
==========================================
+ Coverage   82.66%   82.70%   +0.03%     
==========================================
  Files        1147     1148       +1     
  Lines      446542   447811    +1269     
  Branches   446542   447811    +1269     
==========================================
+ Hits       369154   370376    +1222     
- Misses      54986    54999      +13     
- Partials    22402    22436      +34     

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