Add cel filter for pull request actions in github example

The GitHub interceptor doesn't filter the actions for the `pull_request` event type, which means it will trigger when editing a pull requests' description and other similar actions.

This commit adds a CEL interceptor in order to filter the pull request actions and only trigger when a pull request is opened, synchronized, or reopened.

docs/eventlisteners.md

@@ -327,6 +327,11 @@ spec:
               secretKey: secretToken
             eventTypes:
               - pull_request
+        - cel:
+            filter: >-
+              body.action == 'opened' ||
+              body.action == 'synchronize' ||
+              body.action == 'reopened'
       bindings:
         - ref: github-binding
       template:

examples/github/README.md

@@ -29,7 +29,7 @@ Creates an EventListener that listens for GitHub webhook events.
    -H 'X-GitHub-Event: pull_request' \
    -H 'X-Hub-Signature: sha1=0835c8c5dc317870c4e48659df5f3c53213cd348' \
    -H 'Content-Type: application/json' \
-   -d '{"head_commit":{"id":"master"},"repository":{"url": "https://github.com/tektoncd/triggers"}}' \
+   -d '{"action": "opened", "head_commit":{"id":"master"},"repository":{"url": "https://github.com/tektoncd/triggers"}}' \
    http://localhost:8080
    ```
 

examples/github/github-eventlistener-interceptor.yaml

@@ -14,6 +14,11 @@ spec:
               secretKey: secretToken
             eventTypes:
               - pull_request
+        - cel:
+            filter: >-
+              body.action == 'opened' ||
+              body.action == 'synchronize' ||
+              body.action == 'reopened'
       bindings:
         - ref: github-binding
       template: