Implement AW-credential handling

ActiveWorkflow transmits stored credentials which is a less explicit way of storing and transmitting password than just as Agent's options.

Example of received REST body:

{
  ...
  "options": {
    "username": "albert",
    "password": "einstein"
  },
  "credentials": {
     "secret": "values"
  }
  ...
}

How to implement this?

• TOS Agent needs to know which keys to look for in credentials subtree.
• define custom convention to mark values from options as references to credentials?
• e.g.:

{
  ...
  "options": {
    "username": "albert",
    "password": "{{password}}"
  },
  "credentials": {
     "password": "einstein"
  }
  ...
}