Made DefaultBodyLimit const friendly

[StNicolay]

Derived Copy for DefaultBodyLimit
Made DefaultBodyLimit::max and DefaultBodyLimit::disabled const

Motivation

Making DefaultBodyLimit more const friendly will reduce the code duplication for projects trying to add this layer to multiple endpoints but not to the entire route or router by removing the unnecessary clone calls

Solution

By deriving Copy and making those methods const we allow the following code to be used:

const LIMIT: DefaultBodyLimit = DefaultBodyLimit::disable();

Router::new()
    .route(
        "/files",
        get(file::download)
            .post(file::upload.layer(LIMIT))
            .delete(file::delete)
            .patch(file::modify.layer(LIMIT)),
    )

instead of

let limit = DefaultBodyLimit::disable();


Router::new()
    .route(
        "/files",
        get(file::download)
            .post(file::upload.layer(limit.clone()))
            .delete(file::delete)
            .patch(file::modify.layer(limit.clone())),
    )

[jplatte]

Hey, thanks for the PR!

Can you add a changelog entry for it?