feat: allow arbitrary target settings for local toolchains

Author: rickeylev

python/private/local_runtime_toolchains_repo.bzl

@@ -77,6 +77,8 @@ are registered before `default_runtimes`.
 Note that order matters: it determines the toolchain priority within the
 package.
 """,
+        ),
+        "runtime_target_settings": attr.string_list_dict(
         ),
         "_rule_name": attr.string(default = "local_toolchains_repo"),
     },