chore: Refactor JVM shuffle: Move SpillSorter to top level class and add tests

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Step 1 of N to start cleaning up some of the JVM shuffle code to make it easier to add unit tests

What changes are included in this PR?

• Move SpillSorter to top-level class
• Add basic unit tests
• Fix minor bugs (missing synchronization / no checksum calculated for final partition)

How are these changes tested?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.45299% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.84%. Comparing base (f09f8af) to head (4c9c401).
⚠️ Report is 845 commits behind head on main.

Files with missing lines	Patch %	Lines
...ava/org/apache/spark/shuffle/sort/SpillSorter.java	91.15%	4 Missing and 6 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3081      +/-   ##
============================================
+ Coverage     56.12%   59.84%   +3.72%     
- Complexity      976     1414     +438     
============================================
  Files           119      168      +49     
  Lines         11743    15571    +3828     
  Branches       2251     2583     +332     
============================================
+ Hits           6591     9319    +2728     
- Misses         4012     4944     +932     
- Partials       1140     1308     +168     

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