Postgres RLS + permission controlled database managers (#33)

This PR adds Postgres RLS (trait manager + table manager approach) and permission controlled managers for PostgreSQL.

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Co-authored-by: lukinovec <lukinovec@gmail.com>
Co-authored-by: PHP CS Fixer <phpcsfixer@example.com>

tests/RLS/Etc/Comment.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Stancl\Tenancy\Tests\RLS\Etc;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Stancl\Tenancy\Database\Concerns\BelongsToPrimaryModel;
+use Stancl\Tenancy\Database\Concerns\RLSModel;
+
+class Comment extends Model implements RLSModel
+{
+    use BelongsToPrimaryModel;
+
+    public $guarded = [];
+
+    public $table = 'comments';
+
+    public function getRelationshipToPrimaryModel(): string
+    {
+        return 'post';
+    }
+
+    public function post(): BelongsTo
+    {
+        return $this->belongsTo(Post::class, 'post_id');
+    }
+}