Add NormalizePath middleware

[davidpdrsn]

In axum we're removing support for trailing slash redirects (see tokio-rs/axum#1118 for more details). That means its a little more work for users if they want their routes to be matched both with and without a trailing slash. This middleware addresses that.

Example usage with axum:

// what you'd have to do without this middleware
Router::new()
    .route("/foo", get(handler))
    .route("/foo/", get(handler));

// using the middleware we only have to add one route
NormalizePath::trim_trailing_slash(
    Router::new().route("/foo", get(handler))
);

Note that axum's Router::layer cannot be used since routing happens before any middleware are called, thus the requests would 404. That is something we have to address separately in axum but I still think this middleware makes sense on its own.

[Nehliin]

nice!

[davidpdrsn]

I'll fix CI later in a separate PR.