Automatically configure alertmanager URL for backend instances in read-write deployment mode

Is your feature request related to a problem? Please describe.

From https://github.com/grafana/mimir/pull/3683#issuecomment-1346137630:

I deployed a read-write mode cluster and was surprised I had to configure the Alertmanager URL myself. Given Alertmanager is deployed as part of the backend, I would have expected it to be configured automatically.

Describe the solution you'd like

Ruler is automatically configured to use the built-in Alertmanager by default when running with read-write deployment mode, but this can be overridden if required.

Describe alternatives you've considered

(none)

Additional context

(none)