Periodically flush when using PipeWriter in Json

[BrennanConroy]

Makes PooledByteBufferWriter implement PipeWriter and uses PipeWriter.UnflushedBytes to check whether resumable json converters should let the bytes be flushed.

Fixes #66102 and unblocks dotnet/aspnetcore#55740

Didn't remove IAsyncSerializationBufferWriterContext yet because it would require checking BufferWriter is IDisposable and passing the FlushThreshold value into the serialize functions.

We throw if PipeWriter.CanGetUnflushedBytes == false and state.FlushThreshold > 0 as it could lead to excess buffering for large json payloads. We could consider giving users a way to opt-in to the bad behavior in the future. But currently there is no way to set state.FlushThreshold in the PipeWriter case yet.

[eiriktsarpalis]

Didn't remove IAsyncSerializationBufferWriterContext yet because it would require checking BufferWriter is IDisposable and passing the FlushThreshold value into the serialize functions.

I would recommend removing it, as it doesn't really serve purpose anymore. checking if the writer is IDisposable isn't a huge deal and if that isn't acceptable you could just expose a second IDisposable? disposable parameter on the common workhorse method. FlushThreshold could just be a separate parameter.

[eiriktsarpalis]

Thanks!

[eiriktsarpalis]

One of the new tests seems to have failed once in CI.

[davidfowl]

This change looks really good!