#1992138: Helper issue for "field types as TypedData Plugins" needs FieldItem classes to be able to access their own plugin_id and plugin_definition.
The patch over there currently includes this, but it would be best in its own issue IMO.
This was not doable before #1986802: rename PluginInspectionInterface::getDefinition() to getPluginDefinition(), because PluginInspectionInterface had getDefinition(), which clashed with TypedDataInterface's own unrelated getDefinition().