Ability to set up a custom users table

[danharrin]

This PR addresses #30, #67 and part of #58.

It introduces a new configuration option - filament.auth.guard - which can be set to the name of any guard in your app. The user model provided to that guard must implement Filament\Models\Contracts\FilamentUser and use the Filament\Models\Concerns\IsFilamentUser trait. You may specify a few properties on the custom model to tweak the default behaviour:

• static $filamentAdminColumn can be set to the name of a column that dictates if the user is a Filament admin (superuser).
• static $filamentAvatarColumn can be set to the name of a column that stores the path to the user's avatar.
• static $filamentRolesColumn can be set to the name of a column that stores Filament roles. If none is provided, role functionality is disabled.
• static $filamentUserColumn can be set to the name of a column that dictates if the user can access Filament or not.

These properties are accompanied by methods such as canAccessFilament() and isFilamentAdmin() that can be overwritten to customise the experience further.

Since the user model may now contain extra properties not present in the default UserResource, you may publish and customise it using php artisan vendor:publish --tag=filament-user-resource, and setting the filament.user_resource config to \App\Filament\Resources\UserResource::class.