[9.x] Refactor for missing attributes

[bert-w]

• Move functionality of [9.x] Opt-in Model::preventAccessingMissingAttributes() option #44283 (Opt-in Model::preventAccessingMissingAttributes() option) to a new trait RestrictsAttributes instead of it being spread out in HasAttributes and Illuminate\Database\Eloquent\Model. This increases modularity greatly and allows developers to use these traits themselves.
• Make several changes to the HasAttributes trait so it has no dependencies on the RestrictsAttributes trait (that is: functions from this trait have to be checked first using method_exists() before being called. Not having the RestrictsAttributestrait thus reverts the behaviour to before PR [9.x] Opt-in Model::preventAccessingMissingAttributes() option #44283 )
• Change the order of the if-statement (a && b && c) in throwMissingAttributeExceptionIfApplicable() so it doesn't break on the calls $this->exists && !$this->wasRecentlyMerged since these properties are not part of the trait (and therefore may be undefined depending on where you include this trait). Now, the self::preventsAccessingMissingAttributes() is called first which by default returns false therefore ending the if-statement. This only increases stability since a && b && c can be rearranged in any way, but in this situation there is 1 case that is better being called first.

[taylorotwell]

No tests.