[9.x] Allow specifiying custom messages for Rule objects

[ryangjchandler]

There's not currently any simple way to specify a custom message when validating with a rule object.

Here are the current userland work-arounds and caveats:
Ensure custom rule uses translation strings for message
This would affect all uses of the message. Not very useful for changing the message in a single place.

Extending the rule object with a custom message() method
Not a very nice API. A lot of work for a simple thing.

Interacting with the rule object from a Closure rule and using $fail
Also not very nice. Requires knowledge of the Rule's dependencies, etc (think 3rd party packages).

With the changes in this PR, you can now provide a custom message to the $messages array.

// A rule that would fail...
class Example implements Rule
{

}

$request->validate(rules: [
	'foo' => [new Example]
], messages: [
	Example::class => 'My custom message goes here!'
])

In this scenario, the custom messages provided to $messages would be used when the rule fails instead of the default message.

NOTE: I'm not sure if the simple test I've provided is enough. Should I add tests to cover custom messages that have translation replacements in them too?

[ryangjchandler]

I'm going to add some more tests to this:

• Key-specific messages, i.e. 'foo.' . Example::Class
• Translation replacements

[dennisprudlo]

Love it! Had that recently with the new Enum rule which I used to replace most of the Rule::in() rules. After realizing that I cannot simply modify the messages I switched back to the in-rule.

[taylorotwell]

Thanks!

[taylorotwell]

@ryangjchandler can you make a PR to the docs?

[ryangjchandler]

@taylorotwell Yeah, I can do that tomorrow if that's okay?