java: generate showcase

[zhumin8]

Migration script and librarian logic may need special adjustments for generating this library.
java-showcase used to be part of sdk-platform-java, now directly in google-cloud-java

[zhumin8]

In hermetic build, showcase commit is not specified in generation_config.yaml, instead, it is sourced from the pom.xml file:

1. The Workflow Triggers Showcase Mode
   In hermetic_library_generation.yaml, the job explicitly sets showcase_mode: true when calling the local action ./sdk-platform-java/.github/scripts .

2. The Script Fetches the Version from pom.xml
   The action calls hermetic_library_generation.sh, which sources showcase_utilities.sh.
   • reads the gapic-showcase.version property directly from the file java-showcase/gapic-showcase/pom.xml with grep.
   • clones and checkout repo at the commit

3. It Integrates Protos into the Input Directory Before Generate
   The script copies the schema directory (which contains the protos) from the cloned repository directly into the folder where googleapis was cloned:

   cp -r "${showcase_def_dir}/schema" "${api_def_dir}/"

[zhumin8]

In migrate script, will source version specified in java-showcase/gapic-showcase/pom.xml to populate librarian.yaml sources.
Then after migrated to librarian generate, librarian.yaml becomes the source of truth for showcase version.

These 2 values risks out of sync, should at least introduce a workflow to check these points to the same commit:

• librarian.yaml sources showcase commit
• java-showcase/gapic-showcase/pom.xml gapic-showcase.version property