Enable license header exclusions (#29379)

There are some scenarios where the license on a source file is one that
is compatible with our projects yet we do not want to add the license to
the list of approved license headers (to keep the number of files with
that compatible license contained). This commit adds the ability to
exclude a file from the license check.

buildSrc/src/main/groovy/org/elasticsearch/gradle/precommit/LicenseHeadersTask.groovy

@@ -49,6 +49,14 @@ public class LicenseHeadersTask extends AntTask {
     @Input
     List<String> approvedLicenses = ['Apache', 'Generated']
 
+    /**
+     * Files that should be excluded from the license header check. Use with extreme care, only in situations where the license on the
+     * source file is compatible with the codebase but we do not want to add the license to the list of approved headers (to avoid the
+     * possibility of inadvertently using the license on our own source files).
+     */
+    @Input
+    List<String> excludes = []
+
     /**
      * Additional license families that may be found. The key is the license category name (5 characters),
      * followed by the family name and the value list of patterns to search for.
@@ -95,7 +103,7 @@ public class LicenseHeadersTask extends AntTask {
                for (File dir: dirSet.srcDirs) {
                    // sometimes these dirs don't exist, e.g. site-plugin has no actual java src/main...
                    if (dir.exists()) {
-                       ant.fileset(dir: dir)
+                       ant.fileset(dir: dir, excludes: excludes.join(' '))
                    }
                }
             }