gerrit_plugin: Consistently pass kwargs to java rules

This allows us to pass additional parameters to gerrit_plugin:

  gerrit_plugin(
      name = "secure-config",
      javacopts = ["-Xep:InsecureCryptoUsage:OFF"],
      [...]

Change-Id: Ibcd328fc970fe65b33de789081a07bf0f1a0969f
diff --git a/tools/bzl/plugin.bzl b/tools/bzl/plugin.bzl
index 6e5faf0..de82af8 100644
--- a/tools/bzl/plugin.bzl
+++ b/tools/bzl/plugin.bzl
@@ -30,6 +30,7 @@
     resources = resources,
     deps = provided_deps + deps + GWT_PLUGIN_DEPS_NEVERLINK + PLUGIN_DEPS_NEVERLINK,
     visibility = ['//visibility:public'],
+    **kwargs
   )
 
   static_jars = []
@@ -56,6 +57,7 @@
       resources = list(set(srcs + resources)),
       runtime_deps = deps + GWT_PLUGIN_DEPS,
       visibility = ['//visibility:public'],
+      **kwargs
     )
     genrule2(
       name = '%s-static' % name,