Postgres RLS + permission controlled database managers (#33)

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

---------

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

src/Concerns/ManagesRLSPolicies.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\Concerns;
+
+use Illuminate\Support\Facades\DB;
+use stdClass;
+
+/**
+ * Used for easily dropping RLS policies on tables, primarily in migrations.
+ */
+trait ManagesRLSPolicies
+{
+    /** @return string[] */
+    public static function getRLSPolicies(string $table): array
+    {
+        return array_map(
+            fn (stdClass $policy) => $policy->policyname,
+            DB::select("SELECT policyname FROM pg_policies WHERE tablename = '{$table}' AND policyname LIKE '%_rls_policy%'")
+        );
+    }
+
+    public static function dropRLSPolicies(string $table): int
+    {
+        $policies = static::getRLSPolicies($table);
+
+        foreach ($policies as $policy) {
+            DB::statement('DROP POLICY ? ON ?', [$policy, $table]);
+        }
+
+        return count($policies);
+    }
+}