KeyValuePairSettings - support Serilog.Sinks.Async and other simple wrappers

Please describe the current behavior?

Serilog.Sinks.Async is proving to be a popular way to reduce the latency of non-batched sinks, such as the console and file.

Log.Logger = new LoggerConfiguration()
    .WriteTo.Async(
        a => a.File("logs/myapp.txt", rollingInterval: RollingInterval.Day),
        bufferSize: 1000)
    .CreateLogger()

There’s currently no support for applying the wrapper in XML/key-value settings.

Please describe the expected behavior?

Based on @emrahcetiner’s suggestion in the linked issue; the equivalent of the above would be:

<add key="serilog:using:Async" value="Serilog.Sinks.Async" />
<add key="serilog:using:File" value="Serilog.Sinks.File" />

<!-- This is a regular, direct parameter on `WriteTo.Async()` -->
<add key="serilog:write-to:Async.bufferSize" value="1000" />

<!-- `configure` is an `Action<LoggerSinkConfiguration>`; since `LoggerSinkConfiguration`
        is the basis of the `write-to` syntax, we behave as if the directive to the left
        of `File` is effectively a `write-to` -->
<add key="serilog:write-to:Async.configure:File.path" value="logs/myapp.txt" />
<add key="serilog:write-to:Async.configure:File.rollingInterval" value="Day" />

This is quite similar to how nested actions are handled in Serilog.Settings.Configuration.

The simple syntax doesn’t work for all lambda arguments, i.e. ones with more than a single parameter of a *Configuration type like Serilog.Sinks.Map, but, it could provide a basis for WriteTo.Logger() support:

<add key="serilog:write-to:Logger.configureLogger:write-to:File.path" value="logs.txt" />
<add key="serilog:write-to:Logger.configureLogger:filter:ByIncludingOnly.expression"
  value="@Level = 'Warning'" />

(I’d seriously consider taking a source-breaking change to rename that configureLogger parameter to just configure…)

Ideally, the syntax should be regular and predictable enough for Serilog Analyzer to work with (cc @Suchiman).

Thoughts?