Add configurable polling interval for GitHub API calls (#197)

- Add polling-interval-seconds input parameter to action.yaml (default: 10)
- Update polling logic to use configurable interval
- Add input validation to ensure positive values
- Update README with usage documentation

This allows users to customize how frequently the action polls GitHub API
for approval status, enabling them to reduce API calls or speed up response
times based on their needs.

action.yaml

@@ -44,6 +44,10 @@ inputs:
     description: Whether or not to fail the workflow if the approval is denied
     required: false
     default: 'true'
+  polling-interval-seconds:
+    description: Number of seconds to wait between polling GitHub API for approval status
+    required: false
+    default: '10'
 outputs:
   issue-number:
     description: The number of the issue created