[8.x] Add clean method to model

[bernardwiesner]

Adds a clean method to models.

We already have isClean, so having a clean method on the model makes sense.

Examples:

$user = User::find(1);
$user->foo = 'bar';
$user->isClean(); // false
$user->clean();
$user->isClean(); // true

Use cases:

- Caching attributes

1. Cache some query results on custom attribute on model.
2. Re-use cached attribute throughout code. (if attribute not null, don't query DB)
3. Want to clear cached attributes to re-query DB and re-cache.

- Cleaning model before updating

1. Add custom attribute to model
2. Use custom attribute throughout code
3. Clean model
4. Update some of models original attributes and save (this would fail without clean due to non existing column in DB)

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.