Add support for hashed passwords

[wanieru]

Based on: #873

I added support for both supplying hashed passwords into ftpserver.json, but also a config option hash_plaintext_passwords, which when loading a config will check if any passwords are plaintext, and re-encode the json-file after hashing them.

[wanieru]

With the option to automatically hash plain-text passwords, running the server will neatly transform ftpserver.json like this, without breaking any formatting or changing/adding any other properties:

{
  "version": 1,
  "accesses": [
    {
      "user": "test",
      "pass": "test",
      "fs": "os",
      "params": {
        "basePath": "/tmp"
      }
    }
  ],
  "hash_plaintext_passwords": true,
  "passive_transfer_port_range": {
    "start": 2122,
    "end": 2130
  }
}

↓↓↓

{
  "version": 1,
  "accesses": [
    {
      "user": "test",
      "pass": "$2a$10$xD02IYP4PnlOV1k6wxioluwj2q9JfVxikHho0hRdDcHJCS.iCodHC",
      "fs": "os",
      "params": {
        "basePath": "/tmp"
      }
    }
  ],
  "hash_plaintext_passwords": true,
  "passive_transfer_port_range": {
    "start": 2122,
    "end": 2130
  }
}

[fclairamb]

Thank you @wanieru, I really like how you implemented it.

[wanieru]

Thank you @wanieru, I really like how you implemented it.

Thank you!

[SuperQ]

You might be interested in how we implemented a cache for bcrypt to reduce connection times for repeated access.