fix: break circular reference cycles

[smilkuri]

Issue #, if available:
#1273

Description of changes:
PresignUrlMiddleware and EndpointDiscoveryMiddleware hold a direct $client reference back to the AwsClient through its HandlerList, creating a circular reference cycle that PHP's refcounting cannot collect. Replace $client with \WeakReference in both classes. The client outlives its own handler chain, so the weak reference resolves for the lifetime of the middleware.

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[stobrien89]

Thanks for the contribution- just two comments. I led you down the most invasive path

[stobrien89]

Thanks @smilkuri, looks good. Before merging, we'll need you to push up a changelog entry, which goes in .changes/nextrelease- you'll need to create the nextrelease dir. There you'll need to create a json file of any name (e.g. fix-memory-leak.json) with the following structure:

[
    {
        "type": "enhancement",
        "category": "",
        "description": "<description_of_changes>"
    }
]

The reason we'll use a blank namespace for this is because when empty, the category field defaults to the Aws namespace, which is the most appropriate here because the changes apply to two unrelated classes.

[stobrien89]

Thanks for the contribution, @smilkuri!