Allow comma-separated Client.PreferredCaches and trim quotes

[matyasselmeci]

Pelican Service:

• Client
• Plugin

Right now, listing multiple caches in Client.PreferredCaches (or ending it with a + to fall back to the director) requires using a space as a separator. I can see many instances of people forgetting to quote the space, especially when setting it as an environment variable.

I've also seen people mistakenly quote spaces in Docker env files (unlike shell syntax, an env file in Docker preserves the quotes as-is).

1. Support using a comma as well as a space to separate caches from each other and the final +
2. Trim surrounding quote characters (both ' and ")

I mentioned Client.PreferredCaches specifically because we have a known use case, but it's probably a good idea to do this for all space-separated lists.

[github-actions]

This issue is stale because it has been open for 60 days with no activity.