Issue #1919188 by rvilar, vijaycs85, charmi: Create configuration schemas for openid module.

core/modules/openid/config/schema/openid.schema.yml

+# Schema for the configuration files of the openid module.
+
+openid.settings:
+  type: mapping
+  label: 'OpenID settings'
+  mapping:
+    xri_proxy_resolver:
+      type: string
+      label: 'Proxy resolver for XRI'