Issue #3492930: add xapi ActivityAnswer plugin.

Closes #3492930

modules/lms_xapi_activity/config/activity_answer/xapi/core.entity_form_display.lms_activity._bundle_placeholder_.default.yml

+langcode: en
+status: true
+dependencies:
+  config:
+    - field.field.lms_activity._bundle_placeholder_.field_xapi_package
+    - lms.lms_activity_type._bundle_placeholder_
+  module:
+    - lms_xapi
+id: lms_activity._bundle_placeholder_.default
+targetEntityType: lms_activity
+bundle: _bundle_placeholder_
+mode: default
+content:
+  created:
+    type: datetime_timestamp
+    weight: 10
+    region: content
+    settings: {  }
+    third_party_settings: {  }
+  field_xapi_package:
+    type: lms_xapi
+    weight: 26
+    region: content
+    settings:
+      progress_indicator: throbber
+    third_party_settings: {  }
+  name:
+    type: string_textfield
+    weight: -4
+    region: content
+    settings:
+      size: 60
+      placeholder: ''
+    third_party_settings: {  }
+  uid:
+    type: entity_reference_autocomplete
+    weight: 5
+    region: content
+    settings:
+      match_operator: CONTAINS
+      match_limit: 10
+      size: 60
+      placeholder: ''
+    third_party_settings: {  }
+hidden: {  }