[8.x] Add ability to validate one of multiple date formats

[stevebauman]

Description

This PR adds the ability to provide multiple possible date formats to the date_format validation rule, allowing developers to use one request input for multiple possible date format types.

This PR validates that the input must match one of the possible formats.

Use Case

I'm working on a system where a user can enter a date with a year, or without a year, to signify whether that date is recurring. The formats I'm requiring are Y-m-d (one time) or m-d (recurring).

Currently, I have to create my own validation rule, or use multiple inputs to handle the alternate date formats:

public function rules()
{
    return [
        'date_one_time'  => 'date_format:Y-m-d',
        'date_recurring' => 'date_format:m-d',
    ];
}

This can be simplified with this PR:

public function rules()
{
    return [
        'date' => 'date_format:Y-m-d,m-d',
    ];
}

Thanks so much for your time -- no hard feelings on closure! ❤️