fix: Move Spark constants into its own class.

[jiangmichaellll]

No description provided.

[codecov]

Codecov Report

Merging #127 (d80b381) into master (92cfdfd) will decrease coverage by 0.07%.
The diff coverage is 88.00%.

@@             Coverage Diff              @@
##             master     #127      +/-   ##
============================================
- Coverage     59.52%   59.44%   -0.08%     
- Complexity      129      130       +1     
============================================
  Files            24       25       +1     
  Lines           751      752       +1     
  Branches         31       31              
============================================
  Hits            447      447              
- Misses          296      297       +1     
  Partials          8        8              

Impacted Files	Coverage Δ	Complexity Δ
...le/cloud/pubsublite/spark/PslContinuousReader.java	58.53% <0.00%> (ø)	8.00 <0.00> (ø)
...le/cloud/pubsublite/spark/PslMicroBatchReader.java	86.27% <0.00%> (ø)	11.00 <0.00> (ø)
...om/google/cloud/pubsublite/spark/SparkStructs.java	94.11% <94.11%> (ø)	1.00 <1.00> (?)
...a/com/google/cloud/pubsublite/spark/Constants.java	90.90% <100.00%> (-5.39%)	1.00 <1.00> (ø)
...m/google/cloud/pubsublite/spark/PslSparkUtils.java	97.27% <100.00%> (ø)	28.00 <1.00> (ø)

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[anguillanneuf]

LGTM.