feat: add Persistent Connections extension and update stages

- Introduced a new extension for Persistent Connections, including a detailed description and learning objectives.
- Updated relevant stages to utilize the new extension, ensuring a cohesive learning experience for handling persistent HTTP connections.

Author: ryan-gang

course-definition.yml

@@ -131,6 +131,15 @@ extensions:
 
       [http-compression]: https://en.wikipedia.org/wiki/HTTP_compression
 
+  - slug: "http-persistent-connections"
+    name: "Persistent Connections"
+    description_markdown: |
+      In this challenge extension you'll add [persistent connections][http-persistent-connections] support to your HTTP server implementation.
+
+      Along the way you'll learn about persistent connections, the `Connection` header and more.
+
+      [http-persistent-connections]: https://en.wikipedia.org/wiki/HTTP_persistent_connection
+
 stages:
   # A identifier for this stage, needs to be unique within a course.
   - slug: "at4"
@@ -823,7 +832,10 @@ stages:
     marketing_md: |
       In this stage, you'll add support for encoding the response body using `gzip`.
 
+  # Persistent Connections
+
   - slug: "ag9"
+    primary_extension_slug: "http-persistent-connections"
     name: "Persistent connection"
     difficulty: medium
     description_md: |-
@@ -855,6 +867,7 @@ stages:
       In this stage, you'll add support for persistent HTTP connections, allowing multiple requests to be handled over the same TCP connection.
 
   - slug: "ul1"
+    primary_extension_slug: "http-persistent-connections"
     name: "Multiple persistent connections"
     difficulty: medium
     description_md: |-
@@ -887,6 +900,7 @@ stages:
       In this stage, you'll add support for handling multiple concurrent persistent connections.
 
   - slug: "kh7"
+    primary_extension_slug: "http-persistent-connections"
     name: "Connection closure"
     difficulty: medium
     description_md: |-