Http4s Adapter: use Circe to transform the result to Json

[ghostdogpr]

The current code in Http4sAdapter converts the result to a string, then is parsed into Json. Instead, we could use Circe (which is already in scope in the Http4sAdapter) to convert the result to Json directly. This will take care of edge cases (breaklines, etc) and will be faster.

Here's the existing code:

result <- execute(interpreter, query)
                     .fold(
                       err => s"""{"errors":["${err.toString.replace("\"", "'")}"]}""",
                       result => s"""{"data":$result}"""
                     )
json     <- Task.fromEither(parse(result))

Instead, we can define an Encoder[ResponseValue] and Encoder[ErrorResponse] to convert to Json directly.