docs(examples): add custom either middleware example

[Isvane]

Description

Hey! Opening this PR to add a new example showing how to handle conditional, short-circuiting responses in custom tower middleware.

I’ve set it up under examples/custom-middleware-either with its own Cargo.toml, README.md, and src/main.rs to match the layout of the existing examples.

Note: The code is working but a bit rough around the edges, and my doc comments and the README.md are definitely lacking right now. I couldn't quite find the right words to make it super clear. I'd love some feedback on both the wording and the code during the review.

Motivation

This comes from a recent Discord discussion from Tony, Darius and @jlizen. Because Service forces you to have one concrete Response associated type, you run into a wall if your middleware wants to either bounce a request early (with an error body) or let it pass through to the inner service (with whatever body that service returns)

Solution

I added the custom-middleware-either example. Which contains:

• Using Either<B, Full<Bytes>> to gracefully unify the happy-path body and the early-rejection error body.
• Short-circuiting the future using std::future::ready for the error path.