[3.x] DB users (#382)

* Initial draft

* Apply fixes from StyleCI

* Use CI on master branch too

* Pass correct argument to queued DB creators/deleters

* Apply fixes from StyleCI

* Remove new interface from MySQLDBManager

* Make phpunit run

* Apply fixes from StyleCI

* Fix static property

* Default databaseName

* Use database transactions for creating users & granting permissions

* Apply fixes from StyleCI

* Get old tests to pass

* Apply fixes from StyleCI

* Add tests for PermissionControlledMySQLDatabaseManager

* Apply fixes from StyleCI

* Write test for extra config, fix bug with extra config

* Apply fixes from StyleCI

src/Exceptions/TenantDatabaseUserAlreadyExistsException.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\Exceptions;
+
+use Stancl\Tenancy\Contracts\TenantCannotBeCreatedException;
+
+class TenantDatabaseUserAlreadyExistsException extends TenantCannotBeCreatedException
+{
+    /** @var string */
+    protected $user;
+
+    public function reason(): string
+    {
+        return "Database user {$this->user} already exists.";
+    }
+
+    public function __construct(string $user)
+    {
+        parent::__construct();
+
+        $this->user = $user;
+    }
+}