modify pluginJar to Base64 String in PluginData

[misaya295]

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #4473 (0ca60de) into master (420343f) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master    #4473      +/-   ##
============================================
- Coverage     68.37%   68.37%   -0.01%     
- Complexity     7589     7593       +4     
============================================
  Files          1027     1027              
  Lines         29246    29246              
  Branches       2633     2633              
============================================
- Hits          19998    19996       -2     
- Misses         7673     7674       +1     
- Partials       1575     1576       +1     

Impacted Files	Coverage Δ
.../java/org/apache/shenyu/common/dto/PluginData.java	58.10% <0.00%> (+0.77%)	⬆️
...g/apache/shenyu/admin/transfer/PluginTransfer.java	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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