Implement echo method

https://jmap.io/spec-core.html#the-coreecho-method

In order to test that our infrastructure is correct for the new `jmap` package, you will need to implement a simple echo method that returns exactly the same arguments as it is given.

Request:

{
  "using": [ "urn:ietf:params:jmap:core"],
  "methodCalls": [
    [ "Core/echo", {
      "hello": true,
      "high": 5
    }, "b3ff" ]
  ]
}

Response:

{
  "methodResponses": [
    [ "Core/echo", {
      "hello": true,
      "high": 5
    }, "b3ff" ]
  ],
  "sessionState": "75128aab4b1b"
}

DoD: serving the echo method and add memory integration tests.

We need to be careful also about the way we parse JSON for the echo method to not have potential attacks, and can write some tests about it as well (have a look at https://github.com/nst/JSONTestSuite for example).
You can investigate as well if it's possible to get a lazy json parsing with `JsonTransformers` of the `play-json` library.