Integration with Key module

Add credentials provider login to OpenIDConnect client plugins, to allow storing client credentials in config (default) or via Key module (if installed)

Closes #3373168

config/schema/openid_connect.schema.yml

@@ -59,12 +59,18 @@ openid_connect.client.plugin.*:
   type: mapping
   label: 'OpenID Connect plugin base settings'
   mapping: &base
+    credentials_provider:
+      type: string
+      label: 'Defines mechanism to provide credentials'
     client_id:
       type: string
       label: 'Client ID'
     client_secret:
       type: string
       label: 'Client secret'
+    credentials_key:
+      type: string
+      label: 'Reference to key providing credentials'
     iss_allowed_domains:
       type: string
       label: 'Domains that are allowed to initiate SSO using ISS'