Problem/Motivation
#1959806: Provide a generic 'entity_autocomplete' Form API element introduced a generic 'entity_autocomplete' Form API element. But it hardcodes a single entity type. While linking, users should (optionally) be able to select which entity type they want to link to.
Proposed resolution
Introduce the equivalent of DER (Dynamic Entity Reference), sans the storage/formatter/widget, only the Form API element.
Remaining tasks
TBD
User interface changes
TBD
API changes
TBD