Resolve the cache self test failure when drop privs is enabled

[h2zh]

This PR needs to work together with another PR in the xrdhttp-pelican plugin repo. That was already merged in and picked up by Pelican so you don't need to do any extra steps except pulling a Pelican image after 7.15.

• This PR resolves the failed self test in cache when drop privs is enabled. Origin self test won't be affected.
• A new package self_monitor to resolve the import cycles (xrootd package can't be imported to cache package). This package integrates the entry point for both origin and cache's self test.
• Modified the functions in self test package impacted by the drop privs mode
• Tell the plugin what are the self-test file transplant destinations by setting the environment variables (hardcoded the destination paths)
• The self-test file is named "self-test-*.txt" when it created in its birthplace, but once it is transplanted to the selfTest dir, it is renamed to "self-test-cache-server.txt" and overwrite the previous file. The original file in the birthplace will be deleted after the transplant.

How to test

Run Pelican Cache in drop privs mode by setting the following configs in pelican.yaml

Server:
  DropPrivileges: true
  UnprivilegedUser: pelican

Before this PR, the web UI is unable to open. With this one, the web UI works fine.

Update:
Done in a subsequent PR #2292 : Found logs in Cache logs suggesting other 6 directories needed to set correct permissions. They don't crash the server abruptly though.

WARNING[2025-03-20T17:06:33Z] Failed to open auth database for reload:open /etc/pelican/server-web-passwd: permission denied
WARNING[2025-03-20T17:07:03Z] Failed to load key /etc/pelican/issuer-keys/pelican_generated_1740770124479586092_2096306543.pem: failed to read key file: open /etc/pelican/issuer-keys/pelican_generated_1740770124479586092_2096306543.pem: permission denied
WARNING[2025-03-20T17:07:03Z] Failure during cache director-based health test clean up routine: stat /run/pelican/cache/namespace/pelican/monitoring: permission denied
WARNING[2025-03-20T17:08:03Z] Failure during server TLS maintenance routine: open /etc/pelican/certificates/tls.key: permission denied
ERROR[2025-03-20T17:08:03Z] Failure when generating authfile: Failed to create a generated authfile /run/pelican/xrootd/cache/authfile-cache-generated: open /run/pelican/xrootd/cache/authfile-cache-generated: permission denied
ERROR[2025-03-20T17:08:03Z] Failure when emitting the scitokens.cfg: Failed to create a temporary scitokens file /run/pelican/xrootd/cache/scitokens-generated.cfg.tmp: open /run/pelican/xrootd/cache/scitokens-generated.cfg.tmp: permission denied

[jhiemstrawisc]

LGTM. Thanks!!