Pending tenants: Add getPendingAttributes()

This method lets the user specify default values for custom
non-nullable columns. The primary use case is when the tenants table
has a column like 'slug' and createPending() is called with no
value for 'slug'. This would produce an exception due to the column
having no default value.

Here, getPendingAttributes() can set an initial dummy slug (like a
randomly generated string) before it's overwritten during a pull.

getPendingAttributes() accepts an $attributes array which corresponds
to the attributes passed to createPending(). The array returned from
getPendingAttributes() is ultimately merged with $attributes, so
the user doesn't need to use the $attributes value in
getPendingAttributes(), however it serves to provide more context when
the pending attributes might be dependent on $attributes and therefore
derived from the $attributes actually being used.

Also fixed the `finally` branch in createPending() as it was
potentially referencing the $tenant variable before it was initialized.

tests/Etc/Tenant.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Stancl\Tenancy\Tests\Etc;
 
+use Closure;
 use Stancl\Tenancy\Database\Contracts\TenantWithDatabase;
 use Stancl\Tenancy\Database\Concerns\HasDatabase;
 use Stancl\Tenancy\Database\Concerns\HasDomains;
@@ -16,6 +17,7 @@ use Stancl\Tenancy\Database\Models;
 class Tenant extends Models\Tenant implements TenantWithDatabase
 {
     public static array $extraCustomColumns = [];
+    public static ?Closure $getPendingAttributesUsing = null;
 
     use HasDatabase, HasDomains, HasPending;
 
@@ -23,4 +25,9 @@ class Tenant extends Models\Tenant implements TenantWithDatabase
     {
         return array_merge(parent::getCustomColumns(), static::$extraCustomColumns);
     }
+
+    public static function getPendingAttributes(array $attributes): array
+    {
+        return static::$getPendingAttributesUsing ? (static::$getPendingAttributesUsing)($attributes) : [];
+    }
 }