Leverage `php-http/discovery` to let users choose their http client implementation

[welcoMattic]

Note

Reported from php-llm/llm-chain#318

As it's done in OpenAI PHP Client, it could improve the interoperability to leverage https://github.com/php-http/discovery (and psr/http-client-implementation + psr/http-factory-implementation) to reuse HttpClient installed in users' projects or suggest them one that implements PSR-18 interface.
OpenAI PHP Client choose to not be hard dependent on any other Http Client implementation to let use an already installed Http Client or suggest one, comptible with PSR-18.
I'm opening this issue to replicate this behavior to php-llm/llm-chain, instead of "forcing" users to use symfony/http-client.
First step first is to identify where live hardcoded dependencies to Symfony HttpClient.

[nicolas-grekas]

If async is needed, then PSR 18 cannot be used of course. Httplug could be instead. Then SSE? Multiplexing ? Etc. We need to define what features of http we want to build on.

[chr-hertel]

Async and SSE is a must have