Implement batch API

[ScottG489]

Trello offers an API for batching requests. You basically specify the URL paths as you would normally for each API call and send those paths to the batch endpoint. A full URL to do this would look something like this for example:

https://api.trello.com/1/batch?urls=/boards/123?fields=name,/boards/456/lists?fields=name&cards=open&card_fields=idCard

The response is a list of response objects keyed off each sub-response's code and then the response itself. For example:

[
  {
    "200": [
      {
        "id": "5b7756800e4a7650df54ea64",
        "name": "To Do"
      }
    ]
  },
  {
    "200": [
      {
        "id": "5b7756817af1c20b60196fa8",
        "name": "Done"
      }
    ]
  }
]

The batch endpoint is great for reducing the number of requests you make. It's more efficient and can be critical to staying under their rate limits.

There isn't much documentation, but here's what they have:
https://developer.atlassian.com/cloud/trello/rest/api-group-batch/#api-group-batch

Here's an example of it used in action except just going through fetch_json

I'd like to have this implemented more natively (e.g. adding a batch method to the client) and I'm happy to take a swing at it since I've worked with the Trello API fairly extensively.

Let me know if you have any thoughts on how this should be implemented. Otherwise, I'll open a PR and we can see how it looks.