Implement aiprompt_field submodule

Problem/Motivation

Having prompt as a field, may provide best user experience for #3446530: Integrate with AI interpolator
Basically, user would add a "prompt" field to the entity and interpolate with it.

Steps to reproduce

Proposed resolution

1. Answer questions:
   • What are the requirements of this feature (prompt engineering) for AI Interpolator?
   • Where the prompt will be engineered: field configuration or field widget?
2. Create a submodule aiprompt_field (similar like with submodules aiprompt_content and aiprompt_config which already exist). So "Field" would be just another storage type out of many.
3. Upgrade UI somehow, so that "Segments" UI would work as a field. Examples of options:
   • Upgrade segments form to AJAX
   • Organize prompt inside modal
   • Organize prompt inside off-canvas

Remaining tasks

User interface changes

API changes

Data model changes