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/Article.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Stancl\Tenancy\Tests\RLS\Etc;
+
+use Illuminate\Database\Eloquent\Model;
+use Stancl\Tenancy\Database\Concerns\BelongsToTenant;
+use Stancl\Tenancy\Database\Concerns\RLSModel;
+
+/** Used for testing TraitRLSManager */
+class Article extends Model implements RLSModel
+{
+    use BelongsToTenant;
+
+    protected $guarded = [];
+}