RPM packaging should use config.d by default

[brianhlin]

Pelican Service:

• Client
• Plugin
• Registry
• Director
• Origin
• Cache
• Other (please give the detail)

Is your feature request related to a problem? Please describe.

Ahead of the OSG 24 upgrade, we should settle on a more admin-friendly configuration layout using multiple config.d dirs

Describe the solution you'd like

• Treat /etc/pelican/pelican.yaml as the entrypoint. Should be marked as config but will be overwritten by package updates
• Create /usr/share/pelican/config.d: this is where default config will go and any edits will be destroyed upon package update
• Create /etc/pelican/config.d: this is where admin config will go, which will remain untouched by packaging. It's a good location for providing default, commented-out example configurations as long as they're marked config,noreplace