feat: 添加邮件服务器客户端库的基础功能

- 新增完整的Go客户端库实现，支持邮件服务器API的各种操作
- 实现账户管理、签名管理、邮件发送、审计、配额、通道等功能模块
- 提供ServiceAuth和AppAuth两种认证模式的客户端
- 添加详细的README文档，包含安装指南和使用示例
- 配置.gitignore文件以忽略构建产物和开发工具配置
- 支持分页查询、错误处理和客户端选项配置

client.go

@@ -0,0 +1,153 @@
+package emailcli
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+type Client struct {
+	baseURL      string
+	serviceToken string
+	appKey       string
+	appSecret    string
+	httpClient   *http.Client
+}
+
+type Option func(*Client)
+
+func WithHTTPClient(hc *http.Client) Option {
+	return func(c *Client) { c.httpClient = hc }
+}
+
+func WithTimeout(d time.Duration) Option {
+	return func(c *Client) { c.httpClient.Timeout = d }
+}
+
+// NewServiceClient creates a client authenticated with a service token (management APIs).
+func NewServiceClient(baseURL, serviceToken string, opts ...Option) *Client {
+	c := &Client{
+		baseURL:      strings.TrimRight(baseURL, "/"),
+		serviceToken: serviceToken,
+		httpClient:   &http.Client{Timeout: 30 * time.Second},
+	}
+	for _, o := range opts {
+		o(c)
+	}
+	return c
+}
+
+// NewAppClient creates a client authenticated with AppKey/AppSecret (mail sending API).
+func NewAppClient(baseURL, appKey, appSecret string, opts ...Option) *Client {
+	c := &Client{
+		baseURL:    strings.TrimRight(baseURL, "/"),
+		appKey:     appKey,
+		appSecret:  appSecret,
+		httpClient: &http.Client{Timeout: 30 * time.Second},
+	}
+	for _, o := range opts {
+		o(c)
+	}
+	return c
+}
+
+type APIResponse[T any] struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+	Data    T      `json:"data,omitempty"`
+}
+
+type APIError struct {
+	Code    int
+	Message string
+}
+
+func (e *APIError) Error() string {
+	return fmt.Sprintf("api error %d: %s", e.Code, e.Message)
+}
+
+func (c *Client) url(path string) string {
+	return c.baseURL + path
+}
+
+func (c *Client) setAuth(req *http.Request) {
+	if c.serviceToken != "" {
+		req.Header.Set("Authorization", "Bearer "+c.serviceToken)
+	}
+	if c.appKey != "" {
+		req.Header.Set("X-App-Key", c.appKey)
+		req.Header.Set("X-App-Secret", c.appSecret)
+	}
+}
+
+func doRequest[T any](c *Client, ctx context.Context, method, path string, body interface{}, query url.Values) (T, error) {
+	var zero T
+
+	var bodyReader io.Reader
+	if body != nil {
+		b, err := json.Marshal(body)
+		if err != nil {
+			return zero, fmt.Errorf("marshal body: %w", err)
+		}
+		bodyReader = bytes.NewReader(b)
+	}
+
+	reqURL := c.url(path)
+	if len(query) > 0 {
+		reqURL += "?" + query.Encode()
+	}
+
+	req, err := http.NewRequestWithContext(ctx, method, reqURL, bodyReader)
+	if err != nil {
+		return zero, fmt.Errorf("new request: %w", err)
+	}
+
+	if body != nil {
+		req.Header.Set("Content-Type", "application/json")
+	}
+	c.setAuth(req)
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return zero, fmt.Errorf("do request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	respBody, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return zero, fmt.Errorf("read response: %w", err)
+	}
+
+	var apiResp APIResponse[T]
+	if err := json.Unmarshal(respBody, &apiResp); err != nil {
+		return zero, fmt.Errorf("unmarshal response (status %d): %w\nbody: %s", resp.StatusCode, err, string(respBody))
+	}
+
+	if apiResp.Code != 200 {
+		return zero, &APIError{Code: apiResp.Code, Message: apiResp.Message}
+	}
+
+	return apiResp.Data, nil
+}
+
+func get[T any](c *Client, ctx context.Context, path string, query url.Values) (T, error) {
+	return doRequest[T](c, ctx, http.MethodGet, path, nil, query)
+}
+
+func post[T any](c *Client, ctx context.Context, path string, body interface{}) (T, error) {
+	return doRequest[T](c, ctx, http.MethodPost, path, body, nil)
+}
+
+func put[T any](c *Client, ctx context.Context, path string, body interface{}) (T, error) {
+	return doRequest[T](c, ctx, http.MethodPut, path, body, nil)
+}
+
+func del[T any](c *Client, ctx context.Context, path string) (T, error) {
+	return doRequest[T](c, ctx, http.MethodDelete, path, nil, nil)
+}