Add Lotman purge plugin to containers and finish config plumbing in lotman package

[jhiemstrawisc]

Recent changes in the Lotman package allow cache administrators to define lot purge policies that are meant to be consumed by the Lotman+XRootD purge plugin. However, until some changes are officially ingested in upstream xrootd, the purge plugin currently relies on various patches.

This issue needs two changes:

• The purge plugin should be installed in our dev/prod containers (which requires applying a patch to xrootd)
• The last bit of plumbing should be set up that pipes Pelican-level configuration into the plugin.