no way to set `web.external-url` on Managed Alertmanager

[parkedwards]

Hey there! Thanks very much for this project, it's been very simple to set up.

With self-managed Prometheus/Alertmanager, regardless of the helm chart / operator / managed process approach, there's usually a way to set the -web.external-url runtime flag. I'm having some challenges finding a way to inject this with the GMP operator.

Without being able to set this backlink, our Alertmanager notifications won't contain some way for our responders to link back to the AM alert itself, since like many, we're running Alertmanager from inside of a private k8s cluster