Add a "stand-in" proxy to a deployment

Via @marino-mrc.

When COS is deployed via terraform behind a proxy,

```
| Error: Client Error
│
│ with module.cos.module.tempo.juju_application.s3_integrator,
│ on .terraform/modules/cos.tempo/terraform/tempo/main.tf line 20, in resource "juju_application" "s3_integrator":
│ 20: resource "juju_application" "s3_integrator" {
│
│ Unable to create application, got error: resolving with preferred channel: Post "https://api.charmhub.io/v2/charms/refresh": dial tcp
│ 185.125.188.59:443: i/o timeout
```

We should have a test to confirm model-config and charms utilize proxy envvars correctly.