Add support for .Destructure.AsScalar(Type scalarType) method in KeyValuePairSettings

[tsimbalar]

based on comments from #1174 (comment)

Ideally, calling
new LoggerConfiguration().Destructure.AsScalar(Type scalarType)
should be supported by ReadFrom.KeyValuePairs() with the format :
"serilog:destructure:AsScalar.scalarType" / "qualified type name".

This means :

• making the method available even though it is not an extension method (needs a surrogate method with proper extension-method-ish signature)
• knowing how to handle parameters of type Type when given a qualified type name.

In addition, it would be good to

• check if a Assembly fully qualified type name (TopNamespace.SubNameSpace.ContainingClass, MyAssembly, Version=1.3.0.0, Culture=neutral, PublicKeyToken=b17a5c561934e089) is required, or if we can do with only a Fully qualified type name (TopNamespace.SubNameSpace.ContainingClass) given that using directives already import an assembly (see also KeyValuePairSettings : support non "assembly-qualified" typenames #1065 ).
• see how that would translate in the Serilog.Settings.Configuration side of things ...

[tsimbalar]

tested in https://github.com/tsimbalar/serilog-settings-comparison/blob/master/docs/README.md#custom-destructuring