Add multi-repository installation tokens

[nresare]

Summary

• add POST /installation-token/{github_app}/{owner} for tokens scoped to multiple repositories
• authorize every requested repository and enforce a common permission down-scope
• canonicalize repository selections for safe token caching
• document the endpoint and retain the existing single-repository API

Validation

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

[nresare]

Let's make sure that we remove the RepoScope::All functionality from idcat before landing this