feat(singlejar): Add Log4j plugins cache combiner

[stevebarrau]

In singlejar, add support for combining Log4j2 plugins cache file.

Log4j2 plugins are Java annotations collected by a compiler plugin into .dat files for Log4j2 runtime to find them fast. With correct dependency on a java_plugin, Bazel already runs the java plugin compiler correctly. The behavior is correct on bazel run JAR, but not on _deploy.jar fat jars.

The silent clobbering of files in general, and the example of these Log4j2 .dat files in particular, is discussed in #7330.

• File structure of Log4j2Plugins.dat files
• the test data was generated by a python script, happy to commit it as well
• How to merge Log4j2Plugins.dat?

I tested this fix in my project using Bazel 7.0.2, compiling //src:java_tools_prebuilt.zip, and overriding @remote_java_tools_darwin_arm64 and @remote_java_tools_linux.

[stevebarrau]

@sgowroji what question do I need to answer for "awaiting-user-response" to get removed?

@softprops @jwilliams-ocient from #7330; does this help the issue for you?

[sgowroji]

@stevebarrau Could you please take a look at the failing checks?

[stevebarrau]

@sgowroji PTAL.

[hvadehra]

cc @cushon