feature(protoc-gen-prost-serde): add "feature" option

[aleasims]

This PR adds feature(=<string>) option to add to generated include directives. This is helpful if you want to enable Serde support by a Cargo feature of your crate. With this option used all include-s will look like:

#[cfg(feature = "serde")]
include!("my.generated.file.rs");

[aleasims]

@siennathesane I'd appreciate your opinion on this 🙏

I implemented this cause I wasn't able to find any reasonable workaround for the use case "I want Serde only when "serde" is enabled".

[siennathesane]

I think I can be okay with this. Can you please add an example, or update an example with this? That way folks can understand how it's used.

[aleasims]

@siennathesane added the usage example to existing example code. Also fixed a bug in the implementation I've made 🙈

[siennathesane]

lgtm, thanks for the contribution! I'll release this soon.