chore: remove async-trait

[photovoltex]

Removes the async-trait crate as our MSRV is higher then 1.75, which introduced the feature.

This change is not necessary and sadly doesn't remove async-trait from our build chain completely, as zbus still includes it. I looked into removing async-trait from zbus... but they use it so much, that removing it is a huge task itself.

[Copilot]

Pull Request Overview

This PR removes the async-trait crate dependency from the librespot-metadata module, taking advantage of native async trait support introduced in Rust 1.75. The MSRV is now above this version, making the external crate unnecessary.

Key changes:

• Converted the Metadata trait from using #[async_trait] to native async trait syntax with impl Future return types
• Removed all #[async_trait] annotations from trait implementations across multiple metadata types
• Extracted the common logic from the default get implementation into a new helper function map_request_to_message

Reviewed Changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
metadata/Cargo.toml	Removed async-trait = "0.1" dependency
Cargo.lock	Removed async-trait from librespot-metadata dependencies
metadata/src/lib.rs	Converted Metadata trait to use native async trait syntax, added helper function
metadata/src/track.rs	Removed #[async_trait] annotation from Metadata implementation
metadata/src/album.rs	Removed #[async_trait] annotation from Metadata implementation
metadata/src/artist.rs	Removed #[async_trait] annotation from Metadata implementation
metadata/src/episode.rs	Removed #[async_trait] annotation from Metadata implementation
metadata/src/show.rs	Removed #[async_trait] annotation from Metadata implementation
metadata/src/request.rs	Removed #[async_trait] annotation from MercuryRequest trait
metadata/src/playlist/list.rs	Removed #[async_trait] annotation from Metadata implementation
metadata/src/playlist/annotation.rs	Removed #[async_trait] annotation from Metadata implementation
CHANGELOG.md	Added breaking change entry for async-trait removal

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