This code is being moved from https://github.com/serilog/serilog-sinks-periodicbatching

Fixes #2053, allowing Serilog.Sinks.PeriodicBatching's API to be frozen in its backwards-compatible shape.

This PR makes writing asynchronous, batched sinks as straightforward as regular ILogEventSink. This is achieved with a new interface, IBatchedLogEventSink, which closely mirrors the one exposed by Serilog.Sinks.PeriodicBatching for the past few years:

public interface IBatchedLogEventSink
{
    Task EmitBatchAsync(IReadOnlyCollection<LogEvent> batch);
    Task OnEmptyBatchAsync() { return Task.CompletedTask; }
}

Implementing a batched sink is easy:

class MyBatchedSink: IBatchedLogEventSink
{
    public async Task EmitBatchAsync(IReadOnlyCollection<LogEvent> batch)
    {
        foreach (var logEvent in batch)
        {
            await Console.WriteLineAsync(logEvent.Level);
        }
    }
}

And wiring it in is familiar:

Log.Logger = new LoggerConfiguration()
    .WriteTo.Sink<MyBatchedSink>(new { BatchSizeLimit = 10 })
    .CreateLogger();

Sink packages that need batching behind-the scenes will wrap up the WriteTo.Sink() call in their own configuration methods.

Design notes

System.Threading.Channels dependency: this is needed on older platforms; modern .NET has this library in-the-box, leaving Serilog dependency-free on .NET 6, .NET 8, etc.

WriteTo.Sink() vs WriteTo.BatchedSink() etc: the choice here is part in preparation for a potential IAsyncLogEventSink, which would work in a similar manner, and provide an implementation for a WriteTo.Async() style of API.

BatchingOptions pass by reference rather than callback: while many .NET Core APIs use an options => options.X style of configuration API, Serilog currently does this only when the callback argument supports WriteTo.X() style syntax. Passing the BatchingOptions object directly should keep the usage of callback arguments in the Serilog API consistent.

Migrating from Serilog.Sinks.PeriodicBatching

• IBatchedLogEventSink.EmitBatchAsync() accepts IReadOnlyCollection<LogEvent> instead of IEnumerable<LogEvent>: this makes it possible to access the batch size without enumeration (or needing less-widely-supported enumeration APIs)
• IBachedLogEventSink.OnEmptyBatchAsync() has a default implementation on platforms that support it.
• BatchingSink is non-public; WriteTo.Sink() is used instead of explicitly constructing a wrapper sink
• BatchingOptions calls the Period property BufferingTimeLimit