Plumb Lotman config through to XRootD purge plugin

[jhiemstrawisc]

This finally gives Lotman some teeth in the cache. Now that we're running the correct patched version of xrootd, we can configure XRootD to use the lotman purge plugin.

[jhiemstrawisc]

For the reviewer, I think the bar for approval of this PR is that:
a) When Lotman is disabled, everything functions as normal
b) When Lotman is enabled and the appropriate extra configuration is provided, the cache fully initializes and loads the purge plugin.

To test these, you'll need to make sure you're running in a dev container that was built from a very recent latest-itb tag (within the last two days). We only got the new XRootD patches and purge plugin installed in our containers this week, and both are required for anything to work!

You'll also need to make sure you're running a linux pelican-server binary, which is the only place this lotman code doesn't get compiled out.

After that, you should be able to set:

Cache:
  EnableLotman: true

in your configuration. Doing so must result in an extra line of xrootd configuration being populated, which will look something like

pfc.purgelib libXrdPurgeLotMan.so /var/lib/pelican del exp opp ded

If you want to change the location of the lotman database from its default /var/lib/pelican you can further add to your config:

Lotman:
  DbLocation: <your path>

which should update the xrootd configuration.

If this extra xrootd line doesn't appear when lotman is enabled, or it appears when lotman isn't enabled, something is wrong. The presence of this extra configuration must not prevent the cache from fully initializing/booting.

If all that happens, I think I'm happy at this stage. There may be some extra future work to validate certain types of parameters for Lotman, but as you'll see in a comment, I'm punting on that because it's complicated and XRootD will handle it anyway.

[Saartank]

Found some minor problems and created issues: #1955 and #1956. These can be handled separately and do not need to block this PR from being included in 7.13 release.