… client pom.xml files (#5049)

Supports adding templated markers for client pom.xml as part of migrate
command for Java. For more context on why we are adding this see #4921.

Currently, when running on `google-cloud-java`, it correctly inserts
marker except for a few special cases, captured with debug logging for
now. These needs special treatment, and will fix separately.
In addition to these, a few hybrid libraries java-datastore,
java-spanner, java-storage is also off, they are also special cases that
will be fixed separately.
```
╰─$ migrate .                                                                  
2026/04/07 15:31:27 Warning: multiple proto_library_with_info in google/devtools/containeranalysis/v1/BUILD.bazel, using first
2026/04/07 15:31:27 Debug: skipping library google-cloud-java (SkipGenerate is true)
2026/04/07 15:31:27 Debug: no changes needed for library errorreporting (markers may already exist or dependencies not found)
2026/04/07 15:31:27 Debug: skipping library grafeas (pom.xml not found at /usr/local/google/home/zhumin/repos/testjava2/google-cloud-java/java-grafeas/grafeas/pom.xml)
2026/04/07 15:31:27 Debug: no changes needed for library iam (markers may already exist or dependencies not found)
2026/04/07 15:31:27 Inserted markers in 232 Java client pom.xml files
2026/04/07 15:31:27 Successfully migrated 236 Java libraries
```

Preview full change in [draft
pr](zhumin8/google-cloud-java#3).


Fix #4991