Add a convenience function to decode the Message's json data

[svara]

This PR is adds a convenience function to easily decode the Message's json data.
In addition, it also exposes a JsonDataDecoder structure, where you can set a custom json decoder.
This is useful to converts snake-case keys to camel-case keys.

struct OlympicEventResult: Codable {
    var goldWinner: String
    var silverWinner: String
    var bronzeWinner: String
}

let json = """
{
    "silver_winner": "Sound",
    "gold_winner": "Light",
    "bronze_winner": "Unladen Swallow"
}
"""

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
JsonDataDecoder.appDecoder = decoder

[jayohms]

Looks great, nice addition 👍