Move credentials expiry timeout to the config file

[mukkachaitanya]

Description

The timeout value for the expiry of credentials is hardcoded to 120 minutes.

Expected behavior: The time should be moved to the config file. The user should be able to modify the same accordingly by the use of prefs command.

Actual behavior: The value is currently hardcoded.

[prasadtalasila]

@mukkachaitanya and @rrgodhorus, it might also be beneficial to allow the user to set the timeout by using the following command.
autolabjs prefs changetimeout [ --type session --time <seconds>]

Since this is the very first issue @rrgodhorus is going to work on, he can pace himself using the following steps.

1. Add the following new fields in the default preferences file.

   {
    "supportedLanguages" : [
      "java",
      "c",
      "cpp",
      "cpp14",
      "python2",
      "python3"
    ],
   
    "submission" : {
      "language": "java"
      },
   
    "main_server" : {
      "host": "autolab.bits-goa.ac.in",
      "port": "9000"
    },
   
    "gitlab" : {
      "host": "autolab.bits-goa.ac.in"
    }
   
     "timeouts" : {
        "session" : 7200
    }
   }

2. Read the timeout needed in command-validator.js from preferences file.
3. Write a new sub-command suggested above to change the default session timeout value.