Allow multiple GitHub app Installation IDs

Right now, we can use a single GitHub App Installation for these notifications; if you have other GitHub organizations which have a unique installation ID, you cannot use the GitHub service for all of them.

There are a couple of existing solutions here:

- Use a webhook to manage PR updates, deployments, etc for smaller organizations
- Consolidate app repositories into a single GitHub organization

I am proposing a reasonably simple change allowing the Installation ID to be passed into the template to override the default Installation ID. This would require the installation ID to be attached to an ArgoCD application as info metadata or custom templates for each GitHub organization.

Start by tracing how the GitHub service reads its default Installation ID and how custom templates and ArgoCD application info metadata are handled. Define how a per-application Installation ID overrides the default while preserving existing notifications, then verify the behavior across multiple GitHub organizations.

Written by the indexing model from the issue text.