Add client jar for mapper-extras (#47430)

The rest high level client has a dependency on mapper-extras but the jar
is not published so this commit adds a client jar for this module.

Closes #47413

modules/mapper-extras/build.gradle

@@ -20,4 +20,5 @@
 esplugin {
     description 'Adds advanced field mappers'
     classname 'org.elasticsearch.index.mapper.MapperExtrasPlugin'
+    hasClientJar = true
 }