feat: enable serde derivations for all generated types

[MrBT-nano]

Request: Enable Serde Derivations in vtuber-contracts

To support vtuber-commons (which needs to deserialize configurations from TOML/JSON/YAML into typed structs), we need the generated Rust code in vtuber-contracts to include serde::Serialize and serde::Deserialize derivations.

Proposed Change

Update build.rs in vtuber-contracts to include type attributes for all generated messages:

tonic_build::configure()
    .type_attribute(".", "#[derive(serde::Serialize, serde::Deserialize)]")
    .compile_protos(protos, &["proto"])?;

And add serde as a dependency in vtuber-contracts/Cargo.toml.

Impact

This will allow every downstream service to easily convert between Protobuf messages and other data formats (JSON, TOML, YAML) used for persistent configuration.