[Improvement] reduce compiler warnings

[advancedxy]

What changes were proposed in this pull request?

1. reduce the raw use of parameterized class
2. use Collections.singletonList instead of Arrays.asList for single element
3. add type annotation for some method signature and variable declaration

Why are the changes needed?

This makes javac and IntelliJ's compiler happy

Does this PR introduce any user-facing change?

NO

How was this patch tested?

Existing tests should be sufficient.

[codecov-commenter]

Codecov Report

Merging #46 (c9b9f8d) into master (ae674a6) will not change coverage.
The diff coverage is 50.00%.

@@            Coverage Diff            @@
##             master      #46   +/-   ##
=========================================
  Coverage     56.87%   56.87%           
  Complexity     1207     1207           
=========================================
  Files           152      152           
  Lines          8429     8429           
  Branches        816      816           
=========================================
  Hits           4794     4794           
  Misses         3376     3376           
  Partials        259      259           

Impacted Files	Coverage Δ
.../org/apache/uniffle/common/config/ConfigUtils.java	22.36% <0.00%> (ø)
.../org/apache/uniffle/common/config/RssBaseConf.java	91.80% <0.00%> (ø)
...ava/org/apache/uniffle/server/ShuffleUploader.java	80.98% <ø> (ø)
...rg/apache/uniffle/common/config/ConfigOptions.java	91.37% <100.00%> (ø)
...rg/apache/uniffle/coordinator/CoordinatorConf.java	96.15% <100.00%> (ø)
...a/org/apache/uniffle/server/ShuffleServerConf.java	99.30% <100.00%> (ø)

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[jerqi]

LGTM, thanks for your contribution.