SPIKE Retain metadata of proxied yum packages that are unpublished on the remote

Currently the metadata in yum proxy repositories is overriden by remote repos when cache is invalidated. The issue arises when proxy has cached packages which are no more available on remote but since the metadata gets overriden, the repos are no more able to serve the cached rpms. This could lead to build failures but we dont have enough evidence of that. An investigation is required on how yum repos are used in real world scenarios and why this issue is not raised by the users

This ticket is raised to investigate: NEXUS-15734