Updates XVIZ 2 to include multiple variables per stream

[jlisee]

We are able to have multiple primitives on the same stream each associated with a different object, we want to be able to do that for variables as well.

We might want to provide a metadata setting in the future to specify whether a stream can have a single or multiple values so the declarative UI will have an easier time of it.

• [x] update spec
• [x] update docs
• [x] update builders
• [x] update tests
• [ ] verify support in streetscape.gl

We are going from:

"variables": {
    "/constraints": {
          "values": [1001.3, 1002.3, 1003.3]
    }
}

To:

"variables": {
    "/constraints": {
      "variables": [
        {
          "values": [1001.3, 1002.3, 1003.3]
          "object_id": "1"
        },
        {
          "values": [1001.3, 1002.3, 1003.3]
          "object_id": "2"
        }
      ]
    }

[jlisee]

@Pessimistress do we have streetscape.gl support for this?