Store passwords for PLAIN authentication mechanism

[AlekSi]

What should be done?

For the PLAIN authentication method, we should store the password securely. We should do it in a way similar to MongoDB:

[
  {
    _id: 'test.username',
    userId: UUID('74d81c52-0b4c-4904-8930-7bd1b72f4e04'),
    user: 'username',
    db: 'test',
    credentials: {
      PLAIN: {
        algo: 'argon2id',
        t: int32(3),
        p: int32(4),
        m: int32(65536),
        hash: types.Binary{…},
        salt: types.Binary{…},
      }
    },
    roles: []
  }
]

credentials.PLAIN document is provided by the internal/util/password.PlainHash function.

We should update createUser method to store password.
We should update usersInfo method to return credentials when called with showCredentials: true.
Actually using/validating password for authentication is not in the scope of this issue.

Where?

createUser and usersInfo command handlers.

Definition of Done

• handler updated;
• integration/compatibility tests added/updated;
• spot refactorings done.