Respect `forward_alert_rules` config option in cos-agent

### Bug Description

Cos-proxy has a `forward_alert_rules` config option to conditionally forward alert rules downstream. Cos-agent does not respect this and always sends loki alert rules to databag:
- https://github.com/canonical/cos-proxy-operator/blob/cf9fabb3b2f1457af2989b8a09c407cee44b1d09/lib/charms/grafana_agent/v0/cos_agent.py#L679

This is also true for metric alert rules

### To Reproduce

`juju config cos-proxy forward_alert_rules=false`

Check that there are no alert rules in the databag for `cos-agent` in a deployment:

### Environment

To test for metrics alert rules, you can deploy this:
```
nrpe --- cos-proxy --- opentelemetry-collector
```

I am not aware of any charms which have bundled Loki alert rules, so you would have to add some to a charm and pack/deploy it.

### Relevant log output

```shell
n/a
```

### Additional context

_No response_

Start in lib/charms/grafana_agent/v0/cos_agent.py around line 679 and trace how Loki and metric alert rules are forwarded to the databag. Reproduce with `juju config cos-proxy forward_alert_rules=false`, using the described deployment, and verify that no alert rules are present in the cos-agent databag when forwarding is disabled.

Written by the indexing model from the issue text.