[5.7] Add starts with validation

[timacdonald]

This PR introduces a "starts with" validation rule. The request input value must start with one of the given rule parameters.

Here is an example of how you could use with stripe credentials:

public function rules()
{
    return [
        'stripe_publishable_key' => [
            'starts_with:pk_test_,pk_live_',
        ],
        'stripe_secret_key' => [
            'starts_with:sk_live_,sk_test_',
        ],
    ];
}

Granted this can be done easily with Regex or a custom validation rule already. I'm also conscious that you probably don't want to introduce a bunch of validation rules that are already easily achieved, however I needed this on a couple of projects and thought it might be handy in core if others think it would be useful to them