Add new captcha: cloudflare turnstile (#22369)

Added a new captcha(cloudflare turnstile) and its corresponding
document. Cloudflare turnstile official instructions are here:
https://developers.cloudflare.com/turnstile

Signed-off-by: ByLCY <bylcy@bylcy.dev>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Jason Song <i@wolfogre.com>

modules/turnstile/turnstile.go

@@ -0,0 +1,92 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package turnstile
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"code.gitea.io/gitea/modules/json"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// Response is the structure of JSON returned from API
+type Response struct {
+	Success     bool        `json:"success"`
+	ChallengeTS string      `json:"challenge_ts"`
+	Hostname    string      `json:"hostname"`
+	ErrorCodes  []ErrorCode `json:"error-codes"`
+	Action      string      `json:"login"`
+	Cdata       string      `json:"cdata"`
+}
+
+// Verify calls Cloudflare Turnstile API to verify token
+func Verify(ctx context.Context, response string) (bool, error) {
+	// Cloudflare turnstile official access instruction address: https://developers.cloudflare.com/turnstile/get-started/server-side-validation/
+	post := url.Values{
+		"secret":   {setting.Service.CfTurnstileSecret},
+		"response": {response},
+	}
+	// Basically a copy of http.PostForm, but with a context
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
+		"https://challenges.cloudflare.com/turnstile/v0/siteverify", strings.NewReader(post.Encode()))
+	if err != nil {
+		return false, fmt.Errorf("Failed to create CAPTCHA request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return false, fmt.Errorf("Failed to send CAPTCHA response: %w", err)
+	}
+	defer resp.Body.Close()
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return false, fmt.Errorf("Failed to read CAPTCHA response: %w", err)
+	}
+
+	var jsonResponse Response
+	if err := json.Unmarshal(body, &jsonResponse); err != nil {
+		return false, fmt.Errorf("Failed to parse CAPTCHA response: %w", err)
+	}
+
+	var respErr error
+	if len(jsonResponse.ErrorCodes) > 0 {
+		respErr = jsonResponse.ErrorCodes[0]
+	}
+	return jsonResponse.Success, respErr
+}
+
+// ErrorCode is a reCaptcha error
+type ErrorCode string
+
+// String fulfills the Stringer interface
+func (e ErrorCode) String() string {
+	switch e {
+	case "missing-input-secret":
+		return "The secret parameter was not passed."
+	case "invalid-input-secret":
+		return "The secret parameter was invalid or did not exist."
+	case "missing-input-response":
+		return "The response parameter was not passed."
+	case "invalid-input-response":
+		return "The response parameter is invalid or has expired."
+	case "bad-request":
+		return "The request was rejected because it was malformed."
+	case "timeout-or-duplicate":
+		return "The response parameter has already been validated before."
+	case "internal-error":
+		return "An internal error happened while validating the response. The request can be retried."
+	}
+	return string(e)
+}
+
+// Error fulfills the error interface
+func (e ErrorCode) Error() string {
+	return e.String()
+}