[8.x] Allow passing when callback to Http client retry method

[dmason30]

TL;DR
A little papercut I have, this adds support for passing a $when callback to the Http::retry() method which is already supported by the retry helper that is used to handle the Http client retries. There are no existing tests for Http::retry() so this PR does not contain any.

When Callback already exists in the retry helper
I have just noticed whilst writing this that the $when callback isn't actually documented on the retry helper docs (https://laravel.com/docs/8.x/helpers#method-retry). It is a very useful feature where you can decide whether to continue retrying or to throw the current exception by returning a boolean from the callback:

$response = retry(
    3, // <-- Max tries
    fn () => SomeThrowableAction::make()->execute(), // <-- Code to execute
    400, // <-- Delay between attempts
    fn ($e) => $e instanceof SomeRetryException, // <-- The when callback
 );

Use with Http client
This PR adds the $when support to the Http client, which can be useful if as seen in the example you only want to retry when it gets a connection failure:

$response = \Http::retry(
    3,
    400,
    fn ($e) => $e instanceof ConnectionException, // <-- Added when callback support
 )->get(...);