Manage certificates in `NginxPrometheusExporter` reconciler

### Bug Description

Although Nginx object reconciler [has `_configure_tls()`](https://github.com/canonical/cos-coordinated-workers/blob/main/src/coordinated_workers/nginx.py#L723) which is called in its reconciler, the same piece of logic is missing from `NginxPrometheusExporter`.

In the layer definition, NginxPrometheusExporter [checks whether certificates are on the container's disk](https://github.com/canonical/cos-coordinated-workers/blob/main/src/coordinated_workers/nginx.py#L866) and if they aren't, it uses http to call Nginx. If the certs aren't provided manually by the charm, it will always use http.

We should add a `tls_config_getter` to `NginxPrometheusExporter`'s constructor and have a method to manage certificates run in the reconciler.

### To Reproduce

.

### Environment

.

### Relevant log output

```shell
.
```

### Additional context

_No response_

Start in src/coordinated_workers/nginx.py, comparing the existing _configure_tls() at line 723 with the NginxPrometheusExporter definition and its certificate check near line 866. Trace the reconciler and constructor, then verify that certificates supplied through tls_config_getter are managed so the exporter uses HTTPS instead of falling back to HTTP.

Written by the indexing model from the issue text.