Allow WebUI startup with OAuth2-only administrators

[brianhlin]

Pelican Service:

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

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

In the OSDF, we're responsible for spinning up multiple Pelican services and we'd like to use the WebUI but we have to set up an admin password for each one. To be safe, we should have a different password for each service but that starts to become a management problem in of itself.

Describe the solution you'd like

I'd like to be able to use the WebUI with only a list of OAuth2-based administrators and forego passwords altogether.