Issue #2107243 by amateescu, jibran, larowlan, Xano: Decouple entity reference selection plugins from field definitions

Issue #2368443 by amateescu: Use the new FallbackPluginManagerInterface in ER's selection plugin manager.

Issue #2015701 by klausi, pfrenssen, amateescu: Convert field type to typed data plugin for entity reference module.

Issue #1950632 by effulgentsia, Wim Leers, amateescu, yched: Create a FieldDefinitionInterface, decoupled from  and  config entities, and use it for formatters and widgets.

Issue #1801304 by Amitaibu, amateescu, effulgentsia, tim.plunkett, EclipseGc, Fabianx, Cottser: Added Entity reference field.