Remove our custom `Either`

We have a [custom Either](https://github.com/awslabs/smithy-rs/blob/main/rust-runtime/aws-smithy-http-server/src/plugin/either.rs) for exactly the reasons described in [this PR](https://github.com/tower-rs/tower/pull/637).

Once `tower` `0.5` is released and integrated we should consider reverting to using `tower`s `Either`.