[9.x] Add a new function onlyTrashed

[michaelnabil230]

This PR introduces an "onlyTrashed" validation rule in Exists and Unique.

For example, we want a user to be able to create a new row in the database user, but we want to search in the database where the deleted_at is not null.
And the same since the same function is in the same trait SoftDeletes

Usage:

Validator::make(
    [
        'name' => 'Michael Nabil',
    ],
    [
        'name' => [
            'required', 
            Rule::exists('users')->where('id', $this->id)->onlyTrashed(),  
            Rule::unique('users')->where('id', $this->id)->onlyTrashed(),  // Or
        ],
    ]
);

[martinbean]

The naming seems off on this for me. “Only trashed” reads to me as if it’ll only check soft-deleted records; not omit soft-deleted (trashed) records when doing the exists/unique checking 😕

[s4muel]

The naming seems off on this for me. “Only trashed” reads to me as if it’ll only check soft-deleted records; not omit soft-deleted (trashed) records when doing the exists/unique checking 😕

@martinbean Well, it reads to you just fine, because it really checks only within soft-deleted items 🤯