feat: introduce a simple ULID generator

src/UniqueIdentifierGenerators/ULIDGenerator.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\UniqueIdentifierGenerators;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Support\Str;
+use Stancl\Tenancy\Contracts\UniqueIdentifierGenerator;
+
+/**
+ * Generates a UUID for the tenant key.
+ */
+class ULIDGenerator implements UniqueIdentifierGenerator
+{
+    public static function generate(Model $model): string|int
+    {
+        return Str::ulid()->toString();
+    }
+}