feat: Make shuffle writer buffer size configurable

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

There was no major reason for doing this, but it seems better to have a config rather than hard-coded values.

What changes are included in this PR?

How are these changes tested?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.42%. Comparing base (f09f8af) to head (817e9f0).
⚠️ Report is 767 commits behind head on main.

Files with missing lines	Patch %	Lines
...on/src/main/scala/org/apache/comet/CometConf.scala	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2899      +/-   ##
============================================
+ Coverage     56.12%   59.42%   +3.29%     
- Complexity      976     1368     +392     
============================================
  Files           119      167      +48     
  Lines         11743    15351    +3608     
  Branches       2251     2546     +295     
============================================
+ Hits           6591     9122    +2531     
- Misses         4012     4943     +931     
- Partials       1140     1286     +146     

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[wForget]

Why not use bytesConf?

[comphead]

LGTM thanks @andygrove

Thanks @wForget for the feedback it makes sense. bytesConf can be useful if mostly talking the same levels like GB or MBs.

[wForget]

Thanks @andygrove