update command validation for mode value and use an enum for possible values

[jpdahlke]

This quickly unraveled from a simple errorprone update for an UnusedMethod setMode into addressing a few touch points for this command option that weren't straightforward. This effort attempts to address a few things:

• stop setting and then using an environment variable for mode
• fully annotate the possible modes
• allow mixed case values for mode at the command line
• continue to validate the values provided by the user using picocli methods
• clean up some setup exception handling

[dev-mlb]

Cluster mode seems to be broken...server and feeder start up, but register peer doesn't seem to fire...

2024-08-29T19:25:05.980 [localhost-8001] DEBUG emissary.directory.EmissaryNode -   - Node is standalone, ignoring any peer.cfg and only constructing one rendevous peer with the local node
2024-08-29T19:25:05.980 [localhost-8001] DEBUG emissary.directory.DirectoryPlace -   - Not adding peers in standalone nodes
2024-08-29T19:25:05.980 [localhost-8001] DEBUG emissary.directory.DirectoryPlace -   - Configured 0 rendezvous peers from 1 config entries.