Add proper schema for rocketship_seo third party setting config

Closes #3506165

modules/rocketship_page/config/schema/rocketship_page.schema.yml

+node.type.*.third_party.rocketship_seo:
+  type: mapping
+  label: 'Rocketship SEO Config'
+  mapping:
+    seo_fields:
+      type: sequence
+      label: 'SEO Fields'
+      sequence:
+        type: string
+    seo_title:
+      type: string
+      label: 'SEO Title'
+    seo_description:
+      type: string
+      label: 'SEO Description'