Add a helper getter method for common settings

rules/common_settings.bzl

diff --git a/rules/common_settings.bzl b/rules/common_settings.bzl
index e80ab09..edda079 100644
--- a/rules/common_settings.bzl
+++ b/rules/common_settings.bzl
@@ -98,3 +98,7 @@
     },
     doc = "A string-typed build setting that cannot be set on the command line",
 )
+
+# Extracts the value from an attribute that returns BuildSettingInfo
+def get_build_setting_value(build_setting_attr):
+    return build_setting_attr[BuildSettingInfo].value