ScopeConfigInterface can be more than a string

Reverts parts of a5a98df as it restricts @magentoConfigFixture unnecessarily to strings.

Neither `Magento\Framework\App\Config\ScopeConfigInterface` nor `Magento\Framework\App\Config` have this limitation.

```
    /**
     * Retrieve config value by path and scope
     *
     * @param string $path
     * @param string $scope
     * @param null|int|string $scopeCode
     * @return mixed
     */
    public function getValue(
        $path = null,
        $scope = ScopeConfigInterface::SCOPE_TYPE_DEFAULT,
        $scopeCode = null
    ) {
```

Author: kristof-ringleff

dev/tests/integration/framework/Magento/TestFramework/Annotation/ConfigFixture.php

@@ -71,7 +71,7 @@ protected function _getConfigValue($configPath, $scopeCode = null)
      * @param string|null $scopeCode
      * @return string|null
      */
-    protected function getScopeConfigValue(string $configPath, string $scopeType, string $scopeCode = null): ?string
+    protected function getScopeConfigValue(string $configPath, string $scopeType, string $scopeCode = null)
     {
         $result = null;
         if ($scopeCode !== false) {