feat(client): enhance HTTP client configuration with proxy support (#668)

* feat: enhance HTTP client configuration with proxy support

- Add `ProxyURL` field to `Config` struct with YAML tag
- Implement proxy URL parsing and setting in `getHTTPClient` method
- Add test case for `getHTTPClient` method with custom proxy URL setting
- Include `net/url` package in both `config.go` and `config_test.go` files

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* docs: enhance README with Proxy and OAuth2 Docs

- Remove empty lines from README.md
- Add documentation for proxy configuration in client examples
- Include YAML examples for client using a proxy, custom DNS resolver, OAuth2, and identity-aware proxy configurations in README.md

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* docs: add proxy client

Signed-off-by: appleboy <appleboy.tw@gmail.com>

* Update client/config.go

* Update README.md

* Update client/config_test.go

---------

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Signed-off-by: appleboy <appleboy.tw@gmail.com>
Co-authored-by: TwiN <twin@linux.com>

client/config_test.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"net/http"
+	"net/url"
 	"testing"
 	"time"
 )
@@ -79,3 +80,29 @@ func TestConfig_ValidateAndSetDefaults_withCustomDNSResolver(t *testing.T) {
 		})
 	}
 }
+
+func TestConfig_getHTTPClient_withCustomProxyURL(t *testing.T) {
+	proxyURL := "http://proxy.example.com:8080"
+	cfg := &Config{
+		ProxyURL: proxyURL,
+	}
+	cfg.ValidateAndSetDefaults()
+	client := cfg.getHTTPClient()
+	transport := client.Transport.(*http.Transport)
+	if transport.Proxy == nil {
+		t.Errorf("expected Config.ProxyURL to set the HTTP client's proxy to %s", proxyURL)
+	}
+	req := &http.Request{
+		URL: &url.URL{
+			Scheme: "http",
+			Host:   "www.example.com",
+		},
+	}
+	expectProxyURL, err := transport.Proxy(req)
+	if err != nil {
+		t.Errorf("can't proxy the request %s", proxyURL)
+	}
+	if proxyURL != expectProxyURL.String() {
+		t.Errorf("expected Config.ProxyURL to set the HTTP client's proxy to %s", proxyURL)
+	}
+}