[9.x] Add support to detect dirty encrypted model attributes

[PhiloNL]

Currently, a model always determines that an encrypted attribute is modified when you fill a model, even if the value that is being encrypted is the same. This is because it compares the encrypted string, which changes every time you encrypt something regardless of whether the encrypted value is the same.

This PR will add support to detect dirty encrypted model attributes.

$model = Model::create(['billing_address' => ['street' => 'foo']);

$model->fill(['billing_address' => ['street' => 'foo']);
$model->originalIsEquivalent('billing_address'); // Returns false while it must return true

I wasn't sure if the tests should be part of EloquentModelEncryptedCastingTest or DatabaseEloquentModelTest. If they need to be moved, let me know 🙌🏻

This follows up on #38869 and the question from Taylor #38869 (comment).

[PhiloNL]

Point of discussion:

framework/src/Illuminate/Database/Eloquent/Casts/AsEncryptedCollection.php

Lines 31 to 38 in b9203fc

	public function set($model, $key, $value, $attributes)
	{
	if (! is_null($value)) {
	return [$key => Crypt::encryptString(json_encode($value))];
	}
	return null;
	}

All though this PR fixes the $model->originalIsEquivalent(), $model->isDirty() behavior, calling $model->update() will still update the database with a new encrypted string even though the contents is the same. Preferrably this update is skipped because it's unnecessary.

A possible solution is to add a check to theset() method on the caster and only encrypt if the content is different:

public function set($model, $key, $value, $attributes)
{
    if (! is_null($value)) {
        if(json_encode($model->getOriginal($key)) === json_encode($value)) {
            return [$key => $model->getRawOriginal($key)];
        }

        return [$key => Crypt::encryptString(json_encode($value))];
    }

    return null;
}