Make the promise interface a generic in phpDoc

[apokryfos]

Description
Can we add generic phpDoc blocks for promises?

Example
For example in the promise interface:

<?php

namespace GuzzleHttp\Promise;

/**
 * @template T
 */
interface PromiseInterface
{
   // ...  

   /**
     * @param callable(): T $onFulfilled Invoked when the promise fulfills.
     * @param callable $onRejected  Invoked when the promise is rejected.
     * @return PromiseInterface<T>
     */
    public function then(
        callable $onFulfilled = null,
        callable $onRejected = null
    );

    //...
    /**
     * Resolve the promise with the given value.
     *
     * @param T $value
     *
     * @throws \RuntimeException if the promise is already resolved.
     */
    public function resolve($value);

    /**
     * Waits until the promise completes if possible.
     *
     * Pass $unwrap as true to unwrap the result of the promise, either
     * returning the resolved value or throwing the rejected exception.
     *
     * If the promise cannot be waited on, then the promise will be rejected.
     *
     * @param bool $unwrap
     *
     * @return ($unwrap is true ? T : PromiseInterface)
     *
     * @throws \LogicException if the promise has no wait function or if the
     *                         promise does not settle after waiting.
     */
    public function wait($unwrap = true);
}

I imagine the rest of the classes need to have their docblocks updated to reflect this.

Additional context

I can help to implement this but I'm not too confident on the code base to do it alone.

Let me know what you think of this

[brecht-vermeersch]

Would be a great addition