proto_gazelle: support static mode (stackb#327)

This is supported by the underlying gazelle, just need to update
the runner script.

Author: pcj

rules/proto_gazelle.bzl

@@ -122,7 +122,7 @@ _gazelle_runner = rule(
             default = "",
         ),
         "external": attr.string(
-            values = ["", "external", "vendored"],
+            values = ["", "external", "vendored", "static"],
             default = "",
         ),
         "build_tags": attr.string_list(),