Issue #2680953 by Sam152: Ensure simple_sitemap has appropriate config schema

config/schema/simple_sitemap.schema.yml

+simple_sitemap.settings:
+  type: config_object
+  mapping:
+    settings:
+      type: mapping
+      mapping:
+        max_links:
+          label: 'Max Links'
+          type: integer
+        cron_generate:
+          label: 'Cron Generate'
+          type: boolean
+        remove_duplicates:
+          label: 'Remove Duplicates'
+          type: boolean
+        batch_process_limit:
+          label: 'Batch process limit'
+          type: integer
+    custom:
+      label: 'Custom'
+      type: sequence
+      sequence:
+        type: mapping
+        mapping:
+          path:
+            label: 'Path'
+            type: string
+          priority:
+            label: 'Priority'
+            type: float
+    entity_types:
+      label: 'Entity Types'
+      type: sequence
+      sequence:
+        type: sequence
+        sequence:
+          type: mapping
+          mapping:
+            index:
+              label: 'Index'
+              type: integer
+            priority:
+              label: 'Priority'
+              type: float