Scala SDK: support

We should be supporting GetResult.contentAs[T] with T as case class, without having to use the Couchbase JSON. This is supported in the Java SDK, and should be able to be provided by the Scala SDK. As it stands, it's a step back from Java.

One good option would be to provide instances of JsonDeserializer[A],  for each type of <jsonlibrary>Decoder[A] that is currently supported, through imports. Something like this 

```

trait CirceJsonDeserializer

{    implicit def circeJsonDeserializer[A: Decoder]: JsonDeserializer[A] = ... }

object CirceJsonDeserializer extends CirceJsonDeserializer

```

and importing CirceJsonDeserializer would then allow the existing contentAs[T] method to work.