[12.x] Introduce FluentPromise to allow for cleaner chaining in Pool

[cosmastech]

Guzzle Promises are immutable, which means userland chaining in Pool is not possible since we don't have access to the underlying array.

For example, this will not work to return the string length of each request. It will instead just return the responses.

$response = Http::pool(function (Pool $pool) {
    $pool->as('cosmatech')
        ->get('https://cosmastech.com')
        ->then(
            fn ($response) => strlen($response->body())
        );
    $pool->as('laravel')
        ->get('https://laravel.com')
        ->then(
            fn ($response) => strlen($response->body())
        );
});

dd($response);
/*
array:2 [
  "cosmatech" => Illuminate\Http\Client\Response {
   ...
   },
   "laravel" => Illuminate\Http\Client\Response {
   ...
   }
]
*/

Were this PR to be accepted, then the above will produce:

/*
array:2 [
  "cosmatech" => 9095
  "laravel" => 1422023
]
*/