Support authentication through payas-test module

[ramnivas]

• Introduce a mechanism to specify the requesting user. Perhaps something like:

operation: |
    query($id:Int!) {
        venue(id:$id) {
            name
        }
    } 
variable: |
    {
        "id": 1
    }
user: |
    {
       "sub": 20
       "role": "ROLE_ADMIN"
    }

• Use a random string as the PAYAS_JWT_SECRET env variable and use that to both start the server and compute the JWT token after adding additional data needed to the "user" specified. For example,

JWT header:

{
  "alg": "HS256",
  "typ": "JWT"
}

Payload:

{
  "sub":<user.sub>,
  "iat": <now in seconds since epoch>,
  "exp": <now+one hour in seconds since epoch>,
  "role": <user.role>
}

• Make requests with the header Authorization: Bearer <the computed JWT token>

[fx-chun]

Proposed format looks OK to me, will try to implement as-is