feat: add h2c for http server

[j2rong4cn]

为alist的http服务支持HTTP/2 Cleartext (H2C)协议。明文的HTTP/2协议

• 添加配置选项：scheme > enable_h2c 默认为false

开启后支持nginx的grpc_pass

• https://alist.nn.ci/zh/guide/install/reverse-proxy.html#nginx

location / {
  grpc_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  grpc_set_header X-Forwarded-Proto $scheme;
  grpc_set_header Host $http_host;
  grpc_set_header X-Real-IP $remote_addr;
  # grpc_set_header Range $http_range;
  # grpc_set_header If-Range $http_if_range;
  proxy_redirect off;
  grpc_pass 127.0.0.1:5244;
  # the max size of file to upload
  client_max_body_size 20480m;
}