Add support for transport sharing

[GrahamCampbell]

Guzzle 8 added transport sharing, which lets the built-in handlers share DNS, SSL session, and connection cache state, including persistent cURL share handles on PHP 8.5+ that survive across php-fpm requests. Guzzle 7.11 supports the handler-lifetime subset of the feature. This PR exposes the capability through a new transport_sharing client option with five string modes matching Guzzle's vocabulary: none, handler_prefer, handler_require, persistent_prefer, and persistent_require.

The option's semantics are independent of the installed Guzzle version. The SDK feature-detects what the installed Guzzle supports, degrades the prefer modes gracefully (persistent_prefer falls back to handler-lifetime sharing on Guzzle 7.11+ and to no sharing on older versions), and throws for the require modes when they cannot be honored, so persistent_require is a deployment assertion that connection re-use is actually happening. The option applies when the SDK creates the default HTTP handler, since transport sharing is configured on the Guzzle client constructor rather than per request, and the require modes reject custom handler or http_handler options. Aws\Sdk applies the option to the HTTP handler it shares across the clients it creates. The name transport_sharing mirrors Guzzle's client option; renaming it to http_transport_sharing to group with the other HTTP options would be a two-token change if preferred. A separate commit adds a CI matrix leg pinning Guzzle to >=7.11 <8 so the handler-lifetime degradation path is exercised; it can be dropped if unwanted, since the affected tests skip themselves based on feature detection.

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