[9.x] Improve missing attributes functionality

[bert-w]

This is an improvement for the recently merged #44283.

The following changes are included:

1. I got an error in my own project because I was using the HasAttributes trait for a custom class. However, the following piece of code makes a call to the properties $this->exists and $this->wasRecentlyCreated first which did not exist in my implementation, therefore throwing an "Undefined property" ErrorException:

framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php

Lines 465 to 478 in 0702f55

	protected function throwMissingAttributeExceptionIfApplicable($key)
	{
	if ($this->exists &&
	! $this->wasRecentlyCreated &&
	static::preventsAccessingMissingAttributes()) {
	if (isset(static::$missingAttributeViolationCallback)) {
	return call_user_func(static::$missingAttributeViolationCallback, $this, $key);
	}
	throw new MissingAttributeException($this, $key);
	}
	return null;
	}

That if-statement has now been reordered to first call the static::preventsAccessingMissingAttributes(), and since this is false by default the code will work correctly.

2. I moved all associated code to a new concern so this functionality becomes slightly more scoped instead of being added to the 2k+ lines monsterclass HasAttributes.
3. I updated a couple calls in the Model class where these functions were used, so it can now work without the trait.

[taylorotwell]

I am closing this pull request because it lacks sufficient explanation, tests, or both. It is difficult for us to merge pull requests without these things because the change may introduce breaking changes to the framework.

Feel free to re-submit your change with a thorough explanation of the feature and tests - integration tests are preferred over unit tests. Please include it's benefit to end users; the reasons it does not break any existing features; how it makes building web applications easier, etc.

Thanks!