Implement #[serde(with = "...")] for containers

[cormacrelf]

Fixes #1118, at least the original request. I understand some people wanted finalizers / some other kind of thing, this does not address that but I can't really see how it would be incompatible with doing that as well later. On the de side, this is just another implementation for deserialize_body, like from/try_from/transparent. Edit: indeed, seems compatible with #2891, which runs straight after deserialize_body.

I submit this PR out of light frustration, from the 100 times in the last decade that I have tried to write #[serde(with)] on a container and it hasn't existed. serde(with) modules and functions are fantastic and very reusable, and there's a whole ecosystem built around them, but they come with the silly caveat that they do not work on containers. It's silly because the pattern works on a single-variant enum, which is really just a struct, but not a struct that happens to be a container. It's an attribute that could work anywhere and function identically, but doesn't. So I think this is long overdue.

I came to this specifically after trying to tweak the serialization for a whole lot of bitflags flags. V2 of bitflags changed the serialization format to a string representation "A | B", which I have gripes with as a default, but ok. The only way to change to integer serialization for all of the bitflags is to impl Serialize and impl Deserialize on all of them, which is a lot of boilerplate. There's bitflags-serde-legacy, which would be much easier to use if only #[serde(with)] worked on containers. With this functionality, you can just write a serde-with module that's generic over T: bitflags::Flags, and you're done.

Would need docs at https://serde.rs/container-attrs.html