Use `Either` enum, instead of creating multiple enums and manually implementing traits for each one

[dullbananas]

The messy enum problem mostly applies to enums in the apub crate. For example, SiteOrCommunityOrUser could become Either<ApubSite, Either<ApubCommunity, ApubUser>>. In this case, the activitypub_federation crate would have to implement its traits for Either.

Should be done after #5496 to avoid merge conflicts.

https://docs.rs/either/latest/either/enum.Either.html

[Nutomic]

Good idea this makes sense.

Edit: See LemmyNet/activitypub-federation-rust#139

[dessalines]

Same, I really like this idea.

There's even a few cases in the API, such as GetCommunity or GetPerson, that are {id: Option<CommunityId, name: Option<String>}, that it'd be better to use Either for (depending on whether those can work easily with query params).

[Nutomic]

Relevant serde docs: https://serde.rs/enum-representations.html