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/Post.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Stancl\Tenancy\Tests\RLS\Etc;
+
+use Illuminate\Database\Eloquent\Model;
+use Stancl\Tenancy\Database\Concerns\RLSModel;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+use Stancl\Tenancy\Database\Concerns\BelongsToTenant;
+
+/** Used for testing TraitRLSManager */
+class Post extends Model implements RLSModel
+{
+    use BelongsToTenant;
+
+    public $table = 'posts';
+
+    public $timestamps = false;
+
+    protected $guarded = [];
+
+    public function comments(): HasMany
+    {
+        return $this->hasMany(Comment::class, 'post_id');
+    }
+}