refactor(bpf): Introduce route_decision struct

[jschwinger233]

Background

之前 route() 返回一个 s64 承载 outbound | mark << 8 | must << 40，不好读，不好扩展。为了实现 #818 必须在 route() 结果里带上 redirect_ifindex 和 redirect_dir，之前的 64 bits 无法扩展，这个 pr 作为预备工作引入了 route_decision 结构体方便之后扩展，无业务逻辑改动。

（建议按照 commit review，有个调整缩进的改动让 diff 变得不堪入目，我单独扔到一个 patch）

Checklist

• [ ] The Pull Request has been fully tested
• [ ] There's an entry in the CHANGELOGS
• [ ] There is a user-facing docs PR against https://github.com/daeuniverse/dae

Full Changelogs

• [Implement ...]

Issue Reference

Closes #[issue number]

Test Result