🐛 support multiple registry token scopes

[YusongXiao]

Summary

• Accept repeated scope query parameters on /v2/token.
• Validate and normalize every requested repository scope.
• Include all requested scopes in the generated token access entries.
• Keep existing single-scope token requests compatible.

Motivation

Docker may request source repository pull access together with target repository pull,push access when performing a cross-repository blob mount.

The token endpoint previously deserialized scope as a single optional string, causing repeated scope parameters to fail with 400 Bad Request.

Fixes #1

Testing

• Manually verified a Docker push using cross-repository blob mounting.
• docker build --target builder .
• cargo test --release --target x86_64-unknown-linux-musl