[BUGFIX] Security improvements, error handling, and data calculation fixes

[froemken]

Summary of Changes

• External IP Retrieval: Switched from blocking file_get_contents('http://...') to TYPO3 GeneralUtility::getUrl(self::EXTERNAL_IP_URL) using HTTPS with configurable constant.
• Password Decryption Fallback: Extracted password resolution into resolvePassword() in PleskClientFactory to prevent Undefined variable $password runtime error if decryption fails.
• Plesk Session Security: Restricted the direct "Login to Plesk" session button in ServerWidget to TYPO3 administrator backend users to avoid privilege escalation for non-admin editors.
• DataHandlerHook Fix: Allowed nullable ?array &$incomingFieldArray parameter in DataHandlerHook to prevent fatal TypeError when invalidating records during password policy validation.
• Webspace Calculation & Units: Fixed operator precedence when calculating free disk space in WebspaceDataProvider and integrated DisplayUnit enum with getLabel() method for uppercase chart titles (MB, GB, %).
• Cache Scoping: Scoped runtime cache identifier in PleskSiteService by server protocol, host, and port to avoid cache collisions across multiple Plesk server records.