feat: add support for Encode.value and Decode.value

[njlr]

This PR brings back Decode.value and Encode.value.

These allow JSON to be transformed in and out of a backend-agnostic JSON representation in Thoth.Json.Core.

With these, the user can say "any valid JSON would go here".

For example:

open Thoth.Json.Core


type Message = 
  {
    Sender : string
    Subject : string
    Metadata : Json
  }


[<RequireQualifiedAccess>]
module Message =

  let decode : Decoder<Message> =
    Decode.object 
      (fun get -> 
        {
          Sender = get.Required.Field "sender" Decode.string
          Subject = get.Required.Field "subject" Decode.string
          Metadata = get.Required.Field "metadata" Decode.value
        })

I have marked this as draft because there is a decision to make around integer precision:

• Technically JSON spec allows arbitrarily large/precise numbers.
• Most implementations restrict numbers to be a float.
• To be fully general, we should represent numbers using something like a BigDecimal.
  • However, BigDecimal is not part of the .NET standard libraries.
  • Or perhaps a string... but I expect most users would want a number to work with.
  • Or perhaps we could include our own number representation in the library?
• The current choice is float.
• IDecoderHelpers and IEncoderHelpers, as they stand now, do not support arbitrary numbers.