GH-877: Extract logic for generating executables from Java into scripts

[ascopes]

Part of GH-877, to allow reusing this logic for generating a runnable executable of protoc that is managed via a WASM build of protoc later.

[codecov]

Codecov Report

❌ Patch coverage is 86.89655% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.87%. Comparing base (31a6b09) to head (b8f83d9).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
...nplugin/java/JavaAppToExecutableScriptFactory.java	85.96%	11 Missing and 6 partials ⚠️
...protobufmavenplugin/plugins/JvmPluginResolver.java	91.67%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #887      +/-   ##
==========================================
- Coverage   92.88%   92.87%   -0.00%     
==========================================
  Files          72       73       +1     
  Lines        2034     2047      +13     
  Branches      131      131              
==========================================
+ Hits         1889     1901      +12     
- Misses        104      105       +1     
  Partials       41       41              

Files with missing lines	Coverage Δ
...protobufmavenplugin/plugins/JvmPluginResolver.java	88.10% <91.67%> (+2.10%)	⬆️
...nplugin/java/JavaAppToExecutableScriptFactory.java	85.96% <85.96%> (ø)

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