Add possibility to specify additional build parameters

[trejjam]

This PR allows specifying additional build parameters:

Input:

{
  "definition": {
    "id": 28
  },
  "project": {
    "id": "..."
  },
  "sourceBranch": "refs/heads/master",
  "sourceVersion": "...",
  "reason": 943,
  "parameters": "{\"WORLD\": \"hello\"}"
}

In Azure Pipelines:
bash: env | WORLD -> WORLD=hello

[ghost]

All CLA requirements met.

[tjcorr]

closing this out as this was already implemented in #11