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)

spec/validators/host_validator_spec.rb

@@ -39,12 +39,12 @@
   end
 
   context 'with syntactically invalid host' do
-    let(:value) { '_' }
+    let(:value) { '}' }
 
-    it 'does not add any error' do
+    it 'adds error to the record' do
       validate
 
-      expect(cluster_agent_url_configuration.errors).to be_empty
+      expect(cluster_agent_url_configuration.errors).not_to be_empty
     end
   end
 end