feat(signals): add token-aware shutdown wait

[josecelano]

Summary

• Add signals::cancellation_signal(CancellationToken), an additive shutdown waiter that never subscribes to an OS signal.
• Preserve the existing global_shutdown_signal(), shutdown_signal(), shutdown_signal_with_message(), and Halted APIs.
• Add deterministic coverage for injected-token cancellation and legacy Halted::Normal behavior.
• Bump torrust-server-lib from 0.2.0 to 0.3.0 for the additive public API.

Validation

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo publish --dry-run

Related to torrust/torrust-tracker#2234.

[Copilot]

🟢 Approval recommended

No unresolved review comments; changes are additive and preserve existing APIs.

Pull request overview

Adds a token-aware shutdown waiter while preserving existing signal APIs.

Changes:

• Adds cancellation_signal(CancellationToken) and tests.
• Adds tokio-util and updates the lockfile.
• Bumps torrust-server-lib to 0.3.0.

File summaries

File	Description
src/signals.rs	Adds token cancellation and compatibility tests.
Cargo.toml	Updates dependency and package version.
Cargo.lock	Records dependency graph changes.

Review details

• Files reviewed: 2/3 changed files
• Comments generated: 0
• Review effort level: Lite

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[josecelano]

ACK ea333ec