ensure HTTP requests use proxy env vars (#597)

* fix: ensure HTTP requests use proxy env vars

Updated all instances of `http.Transport` to include the `Proxy` field set to `http.ProxyFromEnvironment`. This ensures that the application respects proxy configuration defined by the `HTTP_PROXY`, `HTTPS_PROXY`, and `NO_PROXY` environment variables.

### Changes:
- Modified `http.Transport` initialization across the codebase to use:
  ```go
  Proxy: http.ProxyFromEnvironment
  ```
- Ensured TLS configurations remain intact by preserving `TLSClientConfig`.

### Why:
- Previously, HTTP requests bypassed proxy settings due to missing configuration in the transport layer.
- This fix enables compatibility with proxied environments, aligning with standard Go behavior.

### Impact:
- All HTTP and HTTPS traffic now adheres to proxy settings.
- Domains listed in `NO_PROXY` bypass the proxy as expected.

### Verification:
- Tested with proxy environment variables set (`HTTP_PROXY`, `HTTPS_PROXY`).
- Verified requests route through the proxy and `NO_PROXY` works as intended.

* reformat with fmt

---------

Co-authored-by: Cormac Doherty <cormac.doherty@ncsc.gov.ie>

cmd/csaf_aggregator/config.go

@@ -284,6 +284,7 @@ func (c *config) httpClient(p *provider) util.Client {
 
 	hClient.Transport = &http.Transport{
 		TLSClientConfig: &tlsConfig,
+		Proxy:           http.ProxyFromEnvironment,
 	}
 
 	client := util.Client(&hClient)