Add support for adding attributes to enum variants

[Kriskras99]

When dealing with serde and serde_as it can be useful to add attributes to enum variants:

#[superstruct(
    variants(V2122, V20, V19, V18),
    variant_attributes(derive(Debug, Serialize, Deserialize, Clone)),
    enum_variant_attributes(serde(borrow))
)]
#[derive(Debug, Serialize, Deserialize, Clone)]
#[serde(untagged)]
pub struct ConfigFilesPath<'a> {
    #[superstruct(only(V20, V2122))]
    #[serde(borrow)]
    pub gachacontent: HipStr<'a>,
    #[superstruct(only(V20, V2122))]
    #[serde(borrow)]
    pub ftuesteps: HipStr<'a>,
    #[superstruct(only(V20))]
    #[serde(borrow)]
    pub anthology: HipStr<'a>,
    #[superstruct(only(V20, V2122))]
    #[serde(borrow)]
    pub objectives: HipStr<'a>,
    #[superstruct(only(V19, V20, V2122))]
    #[serde(borrow)]
    pub playlist: HipStr<'a>,
    #[superstruct(only(V20, V2122))]
    #[serde(borrow)]
    pub portraitborders: HipStr<'a>,
    #[superstruct(only(V2122))]
    #[serde(borrow)]
    pub quickplayrules: HipStr<'a>,
    #[superstruct(only(V20, V2122))]
    #[serde(borrow)]
    pub scheduledquests: HipStr<'a>,
    #[superstruct(only(V18, V19))]
    #[serde(borrow)]
    pub dmconfig: HipStr<'a>,
    #[superstruct(only(V19))]
    #[serde(borrow)]
    pub postcards: HipStr<'a>,
}

This PR does need tests, but I'm not sure how to tests this. Do you have any ideas for testing this?

[michaelsproul]

Sorry this slipped off my radar

To test this I think your example using serde(borrow) could be a good start. We could write a test that only passes (or only compiles) if serde(borrow) is being applied.

[Kriskras99]

Also fell of my radar, I was just using a git dependency in my project.

I've added a test that fails to compile when the enum_variant_attributes line is removed.

[Kriskras99]

I've now also added documentation for specific_enum_variant_attributes and fixed some small issues in the book. It's ready for review now @michaelsproul