[SIGMON-654] extended get_fgc_pm_class_name so it returns classes for self and ext

Context

• task: https://its.cern.ch/jira/browse/SIGMON-654

What was done

• get_fgc_pm_class_name extended

QA

• template link to sonar https://acc-sonar.app.cern.ch/dashboard?branch=SIGMON-654_fgc_ext_pm_class&id=lhc-sm-api
• code was unit tested

Review objectives

• ? assessment of code functionality
• ? assessment of code quality
• ? identifying potential refactorings / logic common to other projects
• ? knowledge sharing
• ? getting help to understand a piece of code

Closes SIGMON-654