Support `payload` field on GitHub Deployment notifications

When creating a deployment with the Github integration, I would like to be able to pass `payload` into the deployment.

As per the [Github docs](https://docs.github.com/en/rest/deployments/deployments?apiVersion=2026-03-10#create-a-deployment):
```
The payload parameter is available for any extra information that a deployment system might need. It is a JSON text field that will be passed on when a deployment event is dispatched.
```

Would a PR that adds `payload` as a parameter that can be passed in from the argocd trigger be considered and accepted?
If so, I will open one.
Thanks

The issue names no files or tests; start by tracing the GitHub integration's deployment creation path and how parameters from the Argo CD trigger are handled. Confirm the existing deployment fields and GitHub API request, then verify that a supplied JSON payload is accepted and forwarded with the deployment.

Written by the indexing model from the issue text.