Support column-specific meta data

[netzwerg]

Support via builders, e.g.:

BooleanColumn.builder(id).putMetaData("anyKey", "anyValue").build()

Or via JSON, e.g.:

"fields": [
  {
      "name": "Serving Size",
      "type": "Double",
      "metaData": { "unit": "g" }
  }
]

Make available as Map<String, String> via Column#getMetaData()