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.

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)

cmd/csaf_checker/processor.go

@@ -430,6 +430,7 @@ func (p *processor) fullClient() util.Client {
 
 	hClient.Transport = &http.Transport{
 		TLSClientConfig: &tlsConfig,
+		Proxy: http.ProxyFromEnvironment,
 	}
 
 	client := util.Client(&hClient)
@@ -460,6 +461,7 @@ func (p *processor) basicClient() *http.Client {
 	if p.cfg.Insecure {
 		tr := &http.Transport{
 			TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
+			Proxy: http.ProxyFromEnvironment,
 		}
 		return &http.Client{Transport: tr}
 	}

cmd/csaf_downloader/downloader.go

@@ -121,6 +121,7 @@ func (d *downloader) httpClient() util.Client {
 
 	hClient.Transport = &http.Transport{
 		TLSClientConfig: &tlsConfig,
+		Proxy: http.ProxyFromEnvironment,
 	}
 
 	client := util.Client(&hClient)

cmd/csaf_downloader/forwarder.go

@@ -106,6 +106,7 @@ func (f *forwarder) httpClient() util.Client {
 
 	hClient.Transport = &http.Transport{
 		TLSClientConfig: &tlsConfig,
+		Proxy: http.ProxyFromEnvironment,
 	}
 
 	client := util.Client(&hClient)

cmd/csaf_uploader/processor.go

@@ -51,6 +51,7 @@ func (p *processor) httpClient() *http.Client {
 
 	client.Transport = &http.Transport{
 		TLSClientConfig: &tlsConfig,
+		Proxy: http.ProxyFromEnvironment,
 	}
 
 	return &client