#3459987: config: Add schema definition

config/schema/field_sample_value.schema.yml

+field.field.*.*.*.third_party.field_sample_value:
+  type: mapping
+  label: 'Field instance settings'
+  mapping:
+    id:
+      type: string
+      label: 'Sample value ID'
+    configuration:
+      type: mapping
+      label: 'Sample value configuration'
+      mapping:
+        prevent_save:
+          type: boolean
+          label: 'Prevent Save'
+        count:
+          type: string
+          label: 'Count'
+        filter_format:
+          type: string
+          label: 'Filter Format'