[9.x] allow Collection random() to accept a callable

[browner12]

this allows the user to, rather than passing an int directly, pass a closure that can reference back to the original Collection in case they need to reference the size of the Collection

a little confusing, so here's an example:

We have a User and Comments. Users can have many Comments. Let's say we want to show 5 random comments from a User on a dashboard.

$comments = $user->comments->random(5);

This probably seems fine, but we run into an issue if the User doesn't have at least 5 comments, and Laravel will throw an InvalidArgumentException. So we want to amend this to say we want 5 comments or the total count of the comments, whichever is smaller. In order to do this currently, you need to use a temporary variable.

$comments = $user->comments;
$comments = $comments->random(min(5, count($comments)));

With this PR you'll now be able to chain this entire call:

$comments = $user->comments->random(fn($items) => min(5, count($items)));

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I'm still a little new to the generics docblocks, so LMK if that needs an adjustment.