Refactor API handlers and proxy logic

- Centralized `getClient` logic into a dedicated function to reduce redundancy.
- Moved proxy initialization to a new utility function `SetProxy` in `internal/util/proxy.go`.
- Replaced `Internal` handler with `CLIHandler` in `server.go` for improved clarity and consistency.
- Removed unused functions and redundant HTTP client setup across the codebase for better maintainability.

internal/util/proxy.go

@@ -0,0 +1,37 @@
+package util
+
+import (
+	"context"
+	"github.com/luispater/CLIProxyAPI/internal/config"
+	"golang.org/x/net/proxy"
+	"net"
+	"net/http"
+	"net/url"
+)
+
+func SetProxy(cfg *config.Config, httpClient *http.Client) (*http.Client, error) {
+	var transport *http.Transport
+	proxyURL, errParse := url.Parse(cfg.ProxyUrl)
+	if errParse == nil {
+		if proxyURL.Scheme == "socks5" {
+			username := proxyURL.User.Username()
+			password, _ := proxyURL.User.Password()
+			proxyAuth := &proxy.Auth{User: username, Password: password}
+			dialer, errSOCKS5 := proxy.SOCKS5("tcp", proxyURL.Host, proxyAuth, proxy.Direct)
+			if errSOCKS5 != nil {
+				return nil, errSOCKS5
+			}
+			transport = &http.Transport{
+				DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+					return dialer.Dial(network, addr)
+				},
+			}
+		} else if proxyURL.Scheme == "http" || proxyURL.Scheme == "https" {
+			transport = &http.Transport{Proxy: http.ProxyURL(proxyURL)}
+		}
+	}
+	if transport != nil {
+		httpClient.Transport = transport
+	}
+	return httpClient, nil
+}