Chore: Refactor QueryPlanSerde, move math exprs in separate file

[kazantsev-maksim]

Which issue does this PR close?

Part of #2019

Rationale for this change

See #2019 for rationale.

What changes are included in this PR?

• Moved maths expressions to separate file (maths.scala)

How are these changes tested?

There are no functional changes, so rely on existing tests.

[andygrove]

There is some overlap between this PR and #2018, but I can rebase #2018 on this PR later, if needed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.36232% with 34 lines in your changes missing coverage. Please review.

Project coverage is 58.67%. Comparing base (f09f8af) to head (861ffd2).
Report is 326 commits behind head on main.

Files with missing lines	Patch %	Lines
.../src/main/scala/org/apache/comet/serde/maths.scala	74.04%	15 Missing and 19 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2027      +/-   ##
============================================
+ Coverage     56.12%   58.67%   +2.54%     
- Complexity      976     1237     +261     
============================================
  Files           119      135      +16     
  Lines         11743    13041    +1298     
  Branches       2251     2379     +128     
============================================
+ Hits           6591     7652    +1061     
- Misses         4012     4168     +156     
- Partials       1140     1221      +81     

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