feat: allow creating own http.RoundTripper for http.Client with HttpRoundTripperProvider when customizing pre-configured http.Transport with HttpTransportCustomizer is not sufficient (#31)

feat: allow creating own http.RoundTripper for http.Client with `HttpRoundTripperProvider` when customizing pre-configured http.Transport with `HttpTransportCustomizer` is not sufficient

docs/configuration.md

@@ -58,7 +58,17 @@ if err != nil {
 
 ## Customizing the HTTP Client
 
-You can provide your own HTTP client configuration using the `HttpTransportCustomizer` callback. This is useful if you need to tweak connection settings like timeouts, idle connection settings, or TLS configurations:
+There are two ways to customize the HTTP client used by the UniFi client:
+1. Using the `HttpTransportCustomizer`.
+2. Using the `HttpRoundTripperProvider`.
+
+Those methods are mutually exclusive, and only one can be used at a time. If both are provided, the `HttpRoundTripperProvider` takes precedence,
+unless it returns `nil`, in which case the `HttpTransportCustomizer` is used if defined (or default transport is used).
+
+### Using `HttpTransportCustomizer`
+
+You can provide your own HTTP client transport configuration using the `HttpTransportCustomizer` callback. This is useful if you need to tweak connection settings like timeouts, idle connection settings, 
+or TLS configurations:
 
 ```go
 c, err := unifi.NewClient(&unifi.ClientConfig{
@@ -76,6 +86,21 @@ if err != nil {
 }
 ```
 
+### Using `HttpRoundTripperProvider`
+
+You can provide your own HTTP client configuration using the `HttpRoundTripperProvider` callback. This is useful if you need to create a custom round tripper, when `http.Transport` is not enough:
+
+```go
+c, err := unifi.NewClient(&unifi.ClientConfig{
+    BaseURL: "https://unifi.localdomain",
+    APIKey: "your-api-key",
+    HttpRoundTripperProvider: func() http.RoundTripper {
+        // Create a custom HTTP Round Tripper instance
+        return &http.Transport{}, nil
+    },
+})
+```
+
 ## Using Interceptors
 
 Interceptors let you hook into the request and response flow. They can be used for logging, metrics, or modifying requests/responses.

unifi/client.go

@@ -54,6 +54,7 @@ Fields:
 	VerifySSL:     When false, disables SSL certificate verification.
 	Interceptors:  A slice of ClientInterceptor implementations that can modify requests and responses.
 	HttpTransportCustomizer:  An optional function to customize the HTTP transport (e.g., for custom TLS settings).
+	HttpRoundTripperProvider: A function that returns a http.RoundTripper for customizing the HTTP client. If both HttpTransportCustomizer and HttpRoundTripperProvider are provided, HttpRoundTripperProvider takes precedence.
 	UserAgent:     The User-Agent header string for outgoing HTTP requests.
 	ErrorHandler:  A custom handler for processing HTTP response errors.
 	UseLocking:    If true, enables internal locking for concurrent request processing.
@@ -68,6 +69,7 @@ type ClientConfig struct {
 	VerifySSL                bool
 	Interceptors             []ClientInterceptor
 	HttpTransportCustomizer  HttpTransportCustomizer
+	HttpRoundTripperProvider func() http.RoundTripper
 	UserAgent                string
 	ErrorHandler             ResponseErrorHandler
 	UseLocking               bool
@@ -152,8 +154,13 @@ func parseBaseURL(base string) (*url.URL, error) {
 }
 
 func newClientFromConfig(config *ClientConfig, v *validator) (*client, error) {
+	var rt http.RoundTripper
 	var err error
 	config.URL = strings.TrimRight(config.URL, "/")
+	if config.HttpRoundTripperProvider != nil {
+		rt = config.HttpRoundTripperProvider()
+	}
+	if rt == nil {
 		transport := &http.Transport{
 			Proxy:           http.ProxyFromEnvironment,
 			TLSClientConfig: &tls.Config{InsecureSkipVerify: !config.VerifySSL},
@@ -163,9 +170,11 @@ func newClientFromConfig(config *ClientConfig, v *validator) (*client, error) {
 				return nil, fmt.Errorf("failed customizing HTTP transport: %w", err)
 			}
 		}
+		rt = transport
+	}
 	httpClient := &http.Client{
 		Timeout:   config.Timeout,
-		Transport: transport,
+		Transport: rt,
 	}
 	if config.APIKey == "" {
 		jar, err := cookiejar.New(&cookiejar.Options{PublicSuffixList: publicsuffix.List})