Support for conditional sinks/enrichers

[nblumhardt]

From the tests:

[Fact]
public void ExpressionsControlConditionalSinks()
{
    var sink = new CollectingSink();
    var logger = new LoggerConfiguration()
        .WriteTo.Conditional("A = 1 or A = 2", wt => wt.Sink(sink))
        .CreateLogger();

    foreach (var a in Enumerable.Range(0, 5))
        logger.Information("{A}", a);

    Assert.Equal(2, sink.Events.Count);
}

[Fact]
public void ExpressionsControlConditionalEnrichment()
{
    var sink = new CollectingSink();
    var logger = new LoggerConfiguration()
        .Enrich.When("A = 1 or A = 2", e => e.WithProperty("B", 1))
        .WriteTo.Sink(sink)
        .CreateLogger();

    foreach (var a in Enumerable.Range(0, 5))
        logger.Information("{A}", a);

    Assert.Equal(2, sink.Events.Count(e => e.Properties.ContainsKey("B")));
}

Requires Serilog 2.9.