[DOP-22429] Prefer camelCase for Spark JDBC params

[dolfinus]

Change Summary

There is a difference between onETL-only fields(if_exists, partitioning_mode) and Spark-only fields (partitionColumn, numPartitions). Highlight this difference by using camelCase for Spark, and snake_case for onETL.

camelCase aliases are already preferred by IDE:

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.50%. Comparing base (05fb290) to head (1d44631).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #352      +/-   ##
===========================================
+ Coverage    92.19%   92.50%   +0.31%     
===========================================
  Files          228      228              
  Lines         9918     9918              
  Branches      1014     1014              
===========================================
+ Hits          9144     9175      +31     
+ Misses         580      557      -23     
+ Partials       194      186       -8     

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