Fix GitHub installation when done by someone else

[BSick7]

When onboarding clients, I attempt to install the Nullstone App into the customer's GitHub organization to grant Nullstone access to their repositories.
This sends a request to the administrator.
Once they complete the operation, Nullstone should see their repositories, but it doesn't.

After I did this recently, I noticed that there isn't a record in org_installation or github_app_installations.

This error showed up in OTEL traces:

github.com/BSick7/go-api/errors.ApiError: An error occurred finalizing the setup of the Github App.

I found this in the logs:

enigma/main/e4c05415097f4cdea72d0bcf89754eef] Mon Mar 23 11:21:50 EDT level=ERROR msg="unable to find the installation id in the list of user installations" method=GET scheme=https host=api.nullstone.io path=/github_installations/callbacks error=<nil>

[BSick7]

Our current process expects the user to install the app and github redirect back to Nullstone.
Instead, this process was circumvented because the user authorized the app directly in github.

To accommodate, GitHub recommends configuring a Webhook URL and secret.
Then, you handle X-GitHub-Event: installation in that endpoint.