Issue #3533404: Create "Default custom text" plugin

Closes #3533404

Fixes|Tasks

• Create a settings form to define a default custom text.
• Add new plugin "Display default custom text" to display the defined default custom text when a field is empty. No needs to define a custom text for each field.
• Add tests for the new plugin.

Tests after deploy

• Go to /admin/config/empty_fields/default_custom_text and define the default custom text.
• Go to the Manage display settings of a content type and select "Display default custom text" in the "Empty value behavior" in the field settings.
• Default custom text will be shown when the field is empty.