Add a 'serde-derive' feature

[daladim]

These changes add a way to have #[derive(serde::Serialize)] and #[derive(serde::Deserialize)] added to types from prost-types.

They may or may not be useful depending on the user needs, and they add a dependency, so they are gated behind a serde-derive feature.

This MR will conflict with #414 , but if you agree to merge both of them I can fix the conflicts if you want

[danburkert]

Hi @daladim, thanks for the PR. I agree with the motivation for this change, but unfortunately this implementation will not be able to land. The files you have modified are auto-generated (see bootstrap.rs). I would like to have a prost-build option that would result in the correct serde derive annotations being added to generated code. Critical to this is that the serde format be compatible with the protobuf json serialization rules. See #75 for the tracking issue.