Make SessionServer an environment property.

[CoreyShupe]

https://github.com/feather-rs/feather/blob/cf650cc0b1ec1da717ee69477e6d44fb6f97f697/feather/datapacks/src/vanilla.rs#L15

If possible there should be vanilla-mimic behavior here to allow for a session server to be defined via environment variables or CLI flags with the current as a default.