Sessions management

New in this version:

• /system http endpoint added, to add and remove sessions (experimental);
• Ability to start without sessions;
• Custom api method to download any supported media: downloadToResponse.
  Example POST request (GET also supported):

  curl --location --request POST '127.0.0.1:9503/api/downloadToResponse' \
  --header 'Content-Type: application/json' \
  --data-raw '{
      "media": {
          "_": "messageMediaDocument",
          "document": {
              "_": "document",
              "id": 5470079466401169993,
              "access_hash": -6754208767885394084,
              "file_reference": {
                  "_": "bytes",
                  "bytes": "AkKdqJkAACnyXiaBgp3M3DfBh8C0+mGKXwSsGUY="
              },
              "date": 1551713685,
              "mime_type": "video/mp4",
              "size": 400967,
              "dc_id": 2
          }
      }
  }'