feat(next): added serialize_extra and deserialize_extra feature flags

[lasantosr]

Summary

This pull request closes #594 by adding two additional feature flags: serialize_extra and deserialize_extra

These feature flags are meant to be manually enabled by downstream crates on any individual crate they might require, leaving the default serialize/deserialize features unaltered.

Checklist

• using conventional commits to hightlight user-facing fixes and features

[mzeitlin11]

This generally looks good, thanks! Probably want to wait for #597 to remove borrowed cases (since some of the extra deserialize derives are on request types which borrow data, tricky to use).

Once #597 is in, I'm hoping the feature story could be even simpler since then request parameters and return types could be treated the same with respect to generation of derives, simplifying everything a ton. Right now there's serialize, deserialize, serialize-extra, and deserialize-extra - hopefully then that could be collapsed into just one serialize and deserialize flag each to enable always deriving those traits.

[lasantosr]

It was my bad, I didn't see the manual implementation of serialize for the types.

I'm closing this PR as it doesn't make much sense, sorry!

[arlyon]

No worries @lasantosr thanks for taking the time :)