Introduce models for Receptive Agents (Part 1)

What does this MR do and why?

This change set introduces the model for Receptive Agents (see &12180).

💡 The commits are suited to be reviewed individually.

This MR is part one of a series of MRs to introduce basic Receptive Agents support in the backend:

1. Introduce models for Receptive Agents (Part 1) (!163142 - merged)
2. Receptive Agents REST API (Part 2) (!163143 - merged)
3. Receptive Agents internal REST API (Part 3) (!163144 - merged)
4. Introduce Cluster Agent URL Configuration Graph... (!163493 - merged)

Refs #463426 (closed) #463427 (closed)