Add data_source support so Dialog Options will be dynamic

Mattermost has data_source field which can be users and channels. We can add one more option which is script. If script is defined at data_source field. OpsTool can execute script to populate options. Script requirements:

1. Opstool need to pass all environment variables to the script and arguments if defined.
2. Script should return json array of options.

Sample Json Output

[
  {
    "text": "Option Text",
    "value" "Option Value 1"
  },
  {
    "text": "Option Text 2",
    "value" "Option Value 2"
  }
]