Handle directories permission denied error in drop privs mode

[h2zh]

This PR is built on top of #2120, which should be reviewed first.

During the development of the above PR, I notice there are several other permissions denied errors happened on several directories/locations, as below:

WARNING[2025-03-17T17:06:33Z] Failed to open auth database for reload:open /etc/pelican/server-web-passwd: permission denied

WARNING[2025-03-17T17:07:03Z] 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

ERROR[2025-03-17T17:08:03Z] 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[2025-03-17T17:08:03Z] 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

WARNING[2025-04-30T13:45:36Z] Failure during xrootd maintenance routine: Failure when opening certificate key pair file to pass to xrootd: open /run/pelican/xrootd/cache/pelican/copied-tls-creds.crt: permission denied

WARNING[2025-03-17T17:07:03Z] Failure during cache director-based health test clean up routine: stat /run/pelican/cache/namespace/pelican/monitoring: permission denied

All of them except the last one are resolved in this PR - I didn't fix the last one because it is not urgent and xrootd already has a purge mechanism to deal with test files cleanup - I created a separate issue for this problem #2265

How to test

If you are on a fresh container, spin up all four Pelican services as usual to set up the initial configs. Then shut them down and restart Pelican services in drop privs mode by setting the following configs in pelican.yaml

Server:
  DropPrivileges: true
  UnprivilegedUser: pelican

Check the logging, search for "permissions denied". There should not be any result except the "cache director-based health test clean up .... permission denied". Also consider doing other basic tests.

[h2zh]

Hey @jhiemstrawisc, when you re-review this PR, I'd recommend taking a commit-by-commit approach for the last three commits, rather than viewing the entire diff at once.

[jhiemstrawisc]

LGTM. Thanks, @h2zh!