Normalize enum serialization strategy

[lukaszzborek]

Some enums are missing the serde rename macro (e.g. SystemSnapshotType, SnapshotCompression), while others use#[serde(rename_all = "lowercase")] instead of snake_case.

All enums should use a consistent serialization strategy

[tmchow]

Submitted #3051 - adds the missing serde rename_all attribute to both snapshot enums.