Support adapters

[kjellmorten]

The old "adapter" should be reintroduced, but this time without the transporter aspect. E.g. a JSON adapter should transform payload and response data from and to the service, but may be paired with any transporter. Also, a service may have several adapters, when that makes sense, and also be applied on specific endpoints.

Configuration:

{
  "id": "example",
  "transporter": "http",
  "adapters": ["json", "rest"],
  "endpoints": [
    {
      "match": { "type": "GET" }
    },
    {
      "match": { "type": "SET" },
      "adapters": ["checksumHeader"]
    }
  ]
}

The available adapters should be provided to Integreat.create() in the same way transporters are.