Add "rendered entity" prompt segment.

Problem/Motivation

Even though views allow to embed any content already, it would be more straightforward to add nodes (or other entities) directly to the prompt.

Proposed resolution

Create "rendered entity" prompt segment, where user would choose:
1) content entity type.
2) content entity (autocomplete form element)
3) display format
4) text format how to prepare output (similar like in views segment).

Remaining tasks

User interface changes

API changes

Data model changes