For symmetry with serilog/serilog-settings-configuration#71
For consistency with the way interfaces parameter are handled

For parameters of configuration methods whose type is an abstract class, the key-value settings format should accept as value the fully-qualified type name of a subclass of this abstract class that has a default constructor.

The typical use-case for this enhancement is Serilog.Sinks.Console whose configuration method looks like this :

public static LoggerConfiguration Console(
	this LoggerSinkConfiguration sinkConfiguration,
	LogEventLevel restrictedToMinimumLevel = LevelAlias.Minimum,
	string outputTemplate = DefaultConsoleOutputTemplate,
	IFormatProvider formatProvider = null,
	LoggingLevelSwitch levelSwitch = null,
	LogEventLevel? standardErrorFromLevel = null,
	ConsoleTheme theme = null)

where ConsoleTheme is defined as public abstract class ConsoleTheme .

Given a class

namespace MyCustom.NameSpace
{
    public class MyCustomTheme:ConsoleTheme
    {
        public MyCustomTheme(){
        }

        // snip ...
    }
}

We should be able to configure the console sink like so :

<configuration>
  <appSettings>
    <add key="serilog:using:Console" value="Serilog.Sinks.Console" />
    <add key="serilog:write-to:Console.theme" value="MyCustom.NameSpace.MyCustomTheme" />

I'm not sure how "qualified" the type name needs to be here, or whether adding using: statements are needed, but the behavior should be similar to the way interfaces are processed

( serilog/serilog-sinks-console#28 seems to mean that name must be assembly-qualified at least in Serilog.Settings.Configuration )