feat: Add support for ProblemDetails extensions

[Syrus666]

What kind of change does this PR introduce?
Updates the ProblemDetails to support extensions to the spec. This behaves the same as the MS implementation of ProblemDetails extensions: https://github.com/aspnet/Mvc/blob/master/src/Microsoft.AspNetCore.Mvc.Core/ProblemDetails.cs#L62

This means additional properties on the response will be populated in the new Extensions property.
I have chosen to not change the behaviour of the Errors property in case it's a breaking change for other users.

What is the current behavior?
Extension problem details extension properties are ignored

What is the new behavior?
Extension properties on the problem details response are populated in to new property

What might this PR break?
It shouldn't cause any regressions with this being additive.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
If you'd like to discuss further I'm "syrus" on reactivex.slack.com

[dnfclas]

All CLA requirements met.

[clairernovotny]

This looks good. What change were you considering for Errors?

[Syrus666]

What I'd considered as an alternative was a breaking change of changing the Errors property to be [JsonExtensionData] IDictionary<string, object> rather than Dictionary<string, string[]> and using that as the property to deserialise other properties in same namespace rather than introducing a new one.

Outside of that being a naughty breaking change you could then have scenarios where the problem details responses they were currently getting were like

{
    "type": "type",
    "title": "title",
    "detail": "detail",
    "status": 500,
    "errors": { 
        "foo":[ "bar" ] 
    },
    "baz": 123
}

Which would mean even forcibly casting the values of the Errors dictionary to an array to make their code compile again would cause problems for them as post this PR Errors would've also now contained baz.

So I thought it safest to stick with the new property.

Or I could be over thinking it and no-one uses the Errors property for anything :D

[clairernovotny]

Thanks for the explanation. I've merged this, but it would still be helpful for any doc updates that describe how the ProblemDetails works. Would be great if you could add another small PR for that?