Add #[serde(with)] for containers

[oblique]

Why this is useful:

• It will enable #[serde_with::serde_as(as = "SomeSerdeWithUtil")] to be applied on a container.

• It can be easily used with code-generators such as prost-build. For example:

    prost_build::Config::new()
        .type_attribute(
            "my_messages.MyMessageType",
            "#[derive(Serialize, Deserialize)] #[serde(with = \"my_message_type\")]",
        )
        .complie_protos(&["my_messages.proto"])
        .unwrap();

[dtolnay]

Thanks for the PR!

I would prefer not to build this into serde_derive. But if it's helpful to have reusable code for producing these 1-line impls, someone could factor the needed parts out into its own macro crate.