Model extending

[alecritson]

This PR looks to build on the current work done for model extending and includes but not limited to the following changes:

• Filament resources use the model contract for DI
• Change to use Laravel's morphMap instead of having concrete classes in the database for polymorphic relationships
• There was an issue with custom models when they extended Lunar ones, the newFactory method didn't like having the return type.
• Added a addDir
• Tweaked guessContractClass to take namespace into account as not all Lunar models (from addons) will have the same namespace
• Added addDirectory method to prevent excessive calls to replace or add methods.
• Updated relationships to use modelClass()
• Static functions should forward to the custom class when called.
• If a custom class name is different to the one it extends i.e. App\Models\MyProduct instead of App\Models\Product it will still use the correct table name.
• Tweaked observe method on models that use HasModelExtending to forward to the custom class even if observe is called on the Lunar model.
• Removed redundant tests and updated docs.

Set as draft for the moment as there is a fair amount of changes so just want to double check before hitting the button.

[glennjacobs]

@alecritson going to need to fix the conflicts before this can be merged.