Overlay support spec doc

Specs/Overlay Support Spec.md

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+# Feature Specification: Overlays Support
+
+## Objective
+Support use of Overlays for enabling developers to enhance existing OpenAPI description files without changing the original file.
+
+## Problem Statement
+Existing OpenAPI documents used for AI plugin creation might lack necessary properties or require modifications for them to provide a high quality AI plugin. Direct editing of the original OpenAPI document is often undesirable or impractical.
+
+## Solution Overview
+Overlays provide a flexible mechanism for extending OpenAPI documents without directly modifying the original files. This allows necessary properties to be added, existing values modified, and the OpenAPI description tailored for effective AI plugin development.
+
+## Functional Requirements
+1. **Overlay File Format Support**
+   - Support overlay files in accordance with the Overlay Specification (v1.0.0), available at [Overlay Specification](https://github.com/OAI/Overlay-Specification/blob/3f398c6/versions/1.0.0.md).
+   - Support both YAML and JSON formats for overlay files.
+
+2. **Overlay Application**
+   - Apply overlay file to an existing OpenAPI document to produce a hybrid of the overlay file and the original OpenAPI description file without changing the original file.
+   - Ensure resulting hybrid OpenAPI document remain valid according to the OpenAPI Specification.
+
+3. **Overlay Operations**
+   - Support addition, modification, and removal of properties to the target OpenAPI document.
+
+## Example: Applying Overlay to Modify OpenAPI Description File
+
+**Original OpenAPI Description File**
+
+```yaml
+openapi: 3.0.0
+info:
+  title: Sample API
+  version: 1.0.0
+paths:
+  /users:
+    get:
+      summary: Get a list of users
+      responses:
+        '200':
+          description: Successful response
+  /users/{id}:
+    get:
+      summary: Get a user by ID
+      parameters:
+        - name: id
+          in: path
+          required: true
+          description: User ID
+      responses:
+        '200':
+          description: Successful response
+```
+
+**Overlay File (overlay.yaml)**
+
+```yaml
+overlay: 1.0.0
+info:
+  title: Sample Overlay
+  version: 1.0.0
+actions:
+  - target: "$"
+    update:
+      info:
+        x-overlay-applied: sample-overlay
+      paths:
+        "/users":
+          get:
+            summary: "Retrieve all users"
+        "/users/{id}":
+          get:
+            parameters:
+              - name: id
+                description: "Unique identifier of the user"
+            responses:
+              '404':
+                description: "User not found"
+```
+
+**Resulting OpenAPI document**
+
+```yaml
+openapi: 3.0.0
+info:
+  title: Sample API
+  version: 1.0.0
+paths:
+  /users:
+    get:
+      summary: Retrieve all users  # Updated summary from overlay
+      responses:
+        '200':
+          description: Successful response
+  /users/{id}:
+    get:
+      summary: Get a user by ID  # Unchanged summary from original description
+      parameters:
+        - name: id
+          in: path
+          required: true
+          description: Unique identifier of the user  # Updated description from overlay
+      responses:
+        '200':
+          description: Successful response
+        '404':
+          description: User not found  # New response added from overlay
+```