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/Database/Concerns/ManagesPostgresUsers.php

@@ -0,0 +1,83 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\Database\Concerns;
+
+use Stancl\Tenancy\Database\DatabaseConfig;
+use Stancl\Tenancy\Database\TenantDatabaseManagers\TenantDatabaseManager;
+
+/**
+ * @method \Illuminate\Database\Connection database()
+ * @mixin TenantDatabaseManager
+ */
+trait ManagesPostgresUsers
+{
+    /**
+     * Grant database/schema and table permissions to the user whose credentials are stored in the passed DatabaseConfig.
+     *
+     * With schema manager, the schema name is stored in the 'search_path' key of the connection config,
+     * but it's still accessible using $databaseConfig->getName().
+     */
+    abstract protected function grantPermissions(DatabaseConfig $databaseConfig): bool;
+
+    public function createUser(DatabaseConfig $databaseConfig): bool
+    {
+        /** @var string $username */
+        $username = $databaseConfig->getUsername();
+        $password = $databaseConfig->getPassword();
+
+        $createUser = ! $this->userExists($username);
+
+        if ($createUser) {
+            $this->database()->statement("CREATE USER \"{$username}\" LOGIN PASSWORD '{$password}'");
+        }
+
+        $this->grantPermissions($databaseConfig);
+
+        return $createUser;
+    }
+
+    public function deleteUser(DatabaseConfig $databaseConfig): bool
+    {
+        /** @var TenantDatabaseManager $this */
+
+        // Tenant DB username
+        $username = $databaseConfig->getUsername();
+
+        // Tenant host connection config
+        $connectionName = $this->database()->getConfig('name');
+        $centralDatabase = $this->database()->getConfig('database');
+
+        // Set the DB/schema name to the tenant DB/schema name
+        config()->set(
+            "database.connections.{$connectionName}",
+            $this->makeConnectionConfig($this->database()->getConfig(), $databaseConfig->getName()),
+        );
+
+        // Connect to the tenant DB/schema
+        $this->database()->reconnect();
+
+        // Delete all database objects owned by the user (privileges, tables, views, etc.)
+        // Postgres users cannot be deleted unless we delete all objects owned by it first
+        $this->database()->statement("DROP OWNED BY \"{$username}\"");
+
+        // Delete the user
+        $userDeleted = $this->database()->statement("DROP USER \"{$username}\"");
+
+        config()->set(
+            "database.connections.{$connectionName}",
+            $this->makeConnectionConfig($this->database()->getConfig(), $centralDatabase),
+        );
+
+        // Reconnect to the central database
+        $this->database()->reconnect();
+
+        return $userDeleted;
+    }
+
+    public function userExists(string $username): bool
+    {
+        return (bool) $this->database()->selectOne("SELECT usename FROM pg_user WHERE usename = '{$username}'");
+    }
+}