MessagePack for C# v4: Round5 - #2291

This round finalizes the approach for async deserialization.

Async deserialization uses a two-pass design:
the first pass scans for message boundaries, and the second pass synchronously deserializes the now-bounded buffer.

True async, meaning a design that checks for and awaits buffer refills in the middle of decoding a value, costs too much performance.
Even granting that .NET 11's async runtime may improve things somewhat.

Adding complex code to compensate for that degradation, such as buffer-size checks or bouncing between async and sync paths, is wasted effort.
Grabbing a buffer and processing it synchronously in one go is the foundation of performance,
and serializers are inherently suited to working that way: message boundaries form well-defined blocks, and a single block is never extremely large.

Since skipping is much lighter than actually reading, the performance concern about being two-pass mostly doesn't materialize.
It is certainly better than true async.

PipeReader's (consumed, examined) model is a natural fit for this kind of processing.
By delegating all the complex buffer management to PipeReader, the scanner that actually finds message boundaries ends up being a small amount of code.

Additionally, when the reader is IsCompleted, the two-pass flow is skipped entirely and the data is deserialized synchronously right away.
This eliminates the overhead in typical situations such as a fully received request body in ASP.NET.

The final API looks like this:

Task<T> DeserializeAsync<T>(PipeReader pipeReader, MessagePackSerializerOptions options, CancellationToken cancellationToken = default)
IAsyncEnumerable<T> DeserializeMessagesAsync<T>(PipeReader pipeReader, MessagePackSerializerOptions options, CancellationToken cancellationToken = default)
IAsyncEnumerable<T> DeserializeElementsAsync<T>(PipeReader pipeReader, MessagePackSerializerOptions options, CancellationToken cancellationToken = default)

DeserializeMessagesAsync streams a sequence of concatenated messages, JSONL-style.
DeserializeElementsAsync streams the elements of a MessagePack array.
These correspond to DeserializeAsyncEnumerable(bool topLevelValues) in System.Text.Json.