Important: #97950 - New "iconIdentifier" option in login providers

See forge#97950

Description

A new option iconIdentifier is added to login providers, which accepts any icon that's available in the Icon Registry.

Example:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['backend']['loginProviders'][1433416747] = [
    'provider' => UsernamePasswordLoginProvider::class,
    'sorting' => 50,
    'iconIdentifier' => 'actions-key',
    'label' => 'LLL:EXT:backend/Resources/Private/Language/locallang.xlf:login.link',
];