Optional serde support for Volume (behind a serde feature flag)

[occuros]

Summary

Please add serde support for the Volume type, gated behind a serde cargo feature so downstream users can serialize/deserialize volumes for saves, configs, and tooling.

Motivation

I’d like to persist Volume in save files and config blobs. bevy reflect could be used, but many crates have certain features gated behind serialize

Proposed solution

•	Add an optional dependency on serde and a serde feature (off by default).
•	Derive Serialize/Deserialize for Volume under that feature