Allow assigning cleanup policies to repositories

[anbuc]

• What are you trying to do?
  Assign a cleanup policy like described here: https://help.sonatype.com/repomanager3/cleanup-policies

• What feature or behavior is this required for?
  Remove no longer necessary packages to ease storage requirements

• How could we solve this issue? (Not knowing is okay!)
  Here is where someone implemented it for the Conan repository type:
  NEXUS-21939 Implemented Cleanup policy for Conan Proxy plug-in nexus-repository-conan#80