Add personal access token login method

[david-uhlig]

This PR adds the login_with_token method to RocketChat::Server to make logging in with a personal access token (PAT) explicit.

Although, you could do so already by calling:

require "rocketchat"

rocket_server = RocketChat::Server.new('http://your.server.address/')
session = RocketChat::Session.new(rocket_server, RocketChat::Token.new(userId: "user-id", authToken: "pat")

This was undocumented, and I think it's worth it having a login method for PATs that behaves similarly to login.

With the PR the code for creating a session from a PAT becomes:

require "rocketchat"

rocket_server = RocketChat::Server.new('http://your.server.address/')
session = rocket_server.login_with_token("user-id", "pat")

Tested against Rocket Chat 7.10.9.

Thanks for making and maintaining this library.