Allow JSON() serialization on custom RecordTypes

[MikeStall]

JSON(First(incident)) fails where incident is a custom derived RecordType.

ErrJSONArg1UnsupportedType

Appear to fail here: https://github.com/microsoft/Power-Fx/blob/930ca694c5017eb410987510710246e49b84244e/src/libraries/Microsoft.PowerFx.Core/Texl/Builtins/Json.cs#L114

[MikeStall]

#2190 seems to do some work on this.