[RFC] Adopts *Writable* BaggageContext (Better)

[ktoso]

This adopts the baggage API in the shape of https://github.com/slashmo/gsoc-swift-baggage-context/pull/34

I have discussed with @fabianfett and @tomerd a little bit about this.

---

This adoption is preferable I believe, because of comments that @adam-fowler raised in https://github.com/slashmo/gsoc-swift-baggage-context/pull/34#discussion_r488416689 In general, this means is also ready for the Context requiring a set/get on the baggage, making lifes easier for other folks.

Specifically, we talked with @fabianfett why the Lambda.Context context type was a class -- since it was immutable, it avoided copying around things a lot. We can achieve the same niceness by making it a CoW type, rather than just a struct. It's a bit of work but worth it in the long run as we get the best of both worlds.

[ktoso]

Whoop I PRed from master based branc... fixing

[tomerd]

closing in inactive PRs, feel free to re-open if still relevant