Allow pseudo fields to have configurable settings

Description of the need As discussed in https://github.com/backdrop/backdrop-issues/issues/2379, there exists a possible need for pseudo fields to have settings that can be configured per display mode. Pseudo fields can be created to display on the edit and display forms, but unlike real fields on display forms, they cannot be assigned settings that can be configured via a 'Configure' button.

Proposed solution Allow pseudo fields to define settings which can be configured via a 'Configure' button on display mode forms.

Alternatives that have been considered A workaround exists, but I think it'd be better for core to provide this functionality natively.