Issue #3447750 by penyaskito: Provide config schema

Closes #3447750

config/schema/media_tableau.schema.yml

+field.formatter.settings.media_tableau:
+  type: mapping
+  label: 'Tableau display format settings'
+  mapping:
+    api_version:
+      type: string
+      label: 'Tableau API version'
+    formatter_class:
+      type: string
+      label: 'Formatter class name'
+    width:
+      type: string
+      label: 'Iframe width'
+    height:
+      type: string
+      label: 'Iframe height'