[MINOR] Replace HashSet with ImmutableSet in configs

[kaijchen]

What changes were proposed in this pull request?

Replace HashSet with ImmutableSet in RssMRConfigs and RssSparkConfigs.

Why are the changes needed?

It's error prone to use mutable collection in public static fields.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

CI.

[codecov-commenter]

Codecov Report

Merging #195 (3f91548) into master (d931e8b) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #195      +/-   ##
============================================
- Coverage     58.40%   58.36%   -0.04%     
+ Complexity     1273     1271       -2     
============================================
  Files           158      158              
  Lines          8446     8448       +2     
  Branches        784      784              
============================================
- Hits           4933     4931       -2     
- Misses         3260     3263       +3     
- Partials        253      254       +1     

Impacted Files	Coverage Δ
.../java/org/apache/hadoop/mapreduce/RssMRConfig.java	75.00% <100.00%> (+8.33%)	⬆️
.../java/org/apache/spark/shuffle/RssSparkConfig.java	96.66% <100.00%> (ø)
...storage/handler/impl/DataSkippableReadHandler.java	81.25% <0.00%> (-3.13%)	⬇️
...org/apache/uniffle/server/ShuffleFlushManager.java	76.66% <0.00%> (-1.67%)	⬇️
...apache/uniffle/coordinator/ApplicationManager.java	83.82% <0.00%> (+0.11%)	⬆️

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[jerqi]

Fail test case is unrelated to this pr. LGTM, thanks @kaijchen ,merged.