fix(util-stream): handle backpressure in ChecksumStream

[UnnaturalCabbage]

Description of changes:
Implemented proper backpressure handling in ChecksumStream.

Previously, the _write() method ignored when push() returned false (indicating the internal buffer reached highWaterMark), which led to uncontrolled memory growth when reads were slower than writes.

This change implements the deferred callback pattern: when push() returns false, we store the callback instead of calling it immediately. When downstream requests more data via _read(), we invoke the pending callback to resume upstream flow.

Backstory

I personally faced this issue while downloading large file from S3 to an ECS Fargate local disk (both in us-east-2). And at some point our service ran out of memory. There is also a related bug ticket for AWS SDK v3 opened by another user, but it was unfortunately closed due to an inability to reproduce the issue back then. From what I can tell, this appeared after responseChecksumValidation was set to WHEN_SUPPORTED as the default for aws s3 sdk v3.

How to reproduce:

I created a repository with an example

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[kuhe]

Thank you for your contribution of this fix.

This fix is available in https://www.npmjs.com/package/@smithy/util-stream/v/4.5.13.

This should be brought into your application(s) by running npm up @smithy/util-stream and confirming with npm ls @smithy/util-stream. This should result in a lockfile update.