Feature: #69416 - Plugins (AbstractPlugin) can load custom language file

See Issue #69416

Description

AbstractPlugin::pi_loadLL takes an optional argument specifying path to a language file. It allows placing language files in other paths like in Extbase structure "Resources/Private/Language". Previously language file had to be located in the directory set in the scriptRelPath property.

Impact

Possibility to put language label files in other paths.