Gather Steam Server info

I am not a gamer, but this protocol came up during work so I wrote a quick gather module for it.

It should work against any dedicated Steam server.  You can find example, public targets at https://www.gametracker.com/search/cs/?.  

Example usage:

```
msf auxiliary(server_info) > use auxiliary/scanner/steam/server_info
msf auxiliary(server_info) > set RHOSTS steam-hlds.vuln.lax.rapid7.com
RHOSTS => steam-hlds.vuln.lax.rapid7.com
msf auxiliary(server_info) > run

[+] 10.4.25.20:27015 found 'Counter-Strike 1.6 Server'
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(server_info) > set VERBOSE true
VERBOSE => true
msf auxiliary(server_info) > run

[*] Sending probes to 10.4.25.20->10.4.25.20 (1 hosts)
[+] 10.4.25.20:27015 found '{:version=>48, :name=>"Counter-Strike 1.6 Server", :map=>"cs_assault", :folder=>"cstrike", :game_name=>"Counter-Strike", :game_id=>10, :bots=>0, :game_version=>"1.1.2.7/Stdio", :type=>"Dedicated", :environment=>"Linux", :visibility=>"public", :VAC=>"secured", :players=>"0/16"}'
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf auxiliary(server_info) > services

Services
========

host        port   proto  name   state  info
----        ----   -----  ----   -----  ----
10.4.25.20  27015  udp    steam  open   ---
- :version: 48
  :name: Counter-Strike 1.6 Server
  :map: cs_assault
  :folder: cstrike
  :game_name: Counter-Strike
  :game_id: 10
  :bots: 0
  :game_version: 1.1.2.7/Stdio
  :type: Dedicated
  :environment: Linux
  :visibility: public
  :VAC: secured
  :players: 0/16

```
Methods commonly used on modules are refactored as module mixins. In this case there aren't more modules using Steam. 

Indeed, the Steam support added to Rex, is mainly providing the features needed to write this module. Allowing to encode and decode some messages. 

So writing a module mixin here would be probably redundant. In summary, I don't see any drawback in the module using directly the Steam implementation in Rex here. But, just in case, let me ping @todb-r7 / @hmoore-r7 / @jlee-r7 in case there is any stopper proceeding in this way!.