…ADDRESS

Previously, these environment variables were hardcoded in the Helm chart
template and could not be overridden via coder.env values. This was a
security concern because binding to 0.0.0.0 exposes these endpoints to
any pod in the cluster.

This change:
- Checks if the user has set CODER_PPROF_ADDRESS or CODER_PROMETHEUS_ADDRESS
  in coder.env before applying the default values
- Updates values.yaml documentation to reflect that these can now be
  overridden (and recommends 127.0.0.1 for security)
- Adds test cases for both override scenarios

Users can now restrict pprof and prometheus to localhost only:

```yaml
coder:
  env:
    - name: CODER_PPROF_ADDRESS
      value: "127.0.0.1:6060"
    - name: CODER_PROMETHEUS_ADDRESS
      value: "127.0.0.1:2112"
```

Fixes #21713