Add support for custom PSR-18 HTTP clients

[const-cloudinary]

Brief Summary of Changes

Lets applications send SDK requests through their own PSR-18 HTTP client, so their logging, retries, proxy, and timeouts apply. Without it, the SDK keeps using its built-in Guzzle client exactly as before.

$cloudinary = new Cloudinary();
$cloudinary->setHttpClient(new Psr18Client(HttpClient::create(), new HttpFactory(), new HttpFactory()));

• Cloudinary::setHttpClient() sets the client for API objects returned by uploadApi(), adminApi(), searchApi(), and searchFoldersApi().
• setHttpClient() on UploadApi, AdminApi, SearchApi, SearchFoldersApi, and AccountApi sets it for objects created directly.
• Cloudinary::setDefaultHttpClient() sets a process-wide default. An object's own client wins over the default, which wins over the built-in client.
• Psr18Handler is a Guzzle handler that forwards each built request to the PSR-18 client and maps PSR-18 network and client errors to ConnectException and RequestException. The public API, including *Async() return types, is unchanged.
• psr/http-client is now a direct requirement (already installed through Guzzle). symfony/http-client is added to require-dev.
• CI adds PHP 8.5, and one PHP 8.5 + Guzzle 8 job runs the full suite through the Symfony PSR-18 client.
• Tests call ReflectionMethod::setAccessible() only before PHP 8.1; PHP 8.5 deprecates it, which failed the run.
• Docs: docs/use-custom-http-client.md, examples/use-custom-http-client.php, and a README section.

What does this PR address?

• GitHub issue (Add reference - Drop Guzzle requirement and replace with any PSR-18 HttpClient #396)
• Refactoring
• New feature
• Bug fix
• Adds more tests

Are tests included?

• Yes
• No

Reviewer, please note:

• With a custom client, timeout, connection_timeout, and api_proxy are configured on the client, not in Cloudinary configuration.
• PSR-18 is sync-only, so *Async() calls made through a custom client run one at a time.
• Every request carries an Authorization header with the API secret. The docs tell users to redact it if their client logs headers.
• Drop Guzzle requirement and replace with any PSR-18 HttpClient #396 also asks to drop the Guzzle requirement. This PR keeps Guzzle: requests are still built by Guzzle and *Async() returns Guzzle promises.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I ran the full test suite before pushing the changes and all of the tests pass.