Expose AppEntry for more friendly ergonomics/further impl capabilities

[Wolftousen]

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Expected Behavior

AppEntry is publicly exposed so that App can be further implemented.

Current Behavior

Currently, AppEntry is not exposed publicly and makes the following impossible:

//several routes defined above

pub trait LibARouter {
    fn register_liba_routes(self) -> Self;
}

impl LibARouter for App<AppEntry> {
    fn register_liba_routes(self) -> Self {
        self.service(route_a)
            .service(route_b)
            .service(route_c)
    }
}

Context

I'm currently working on a project to build a rather robust REST API and I'm splitting my route handler functions out into several files. We'll be expanding out to more routes in the future and we want to have minimal changes to the App::new() code. So we thought about doing the code snippet above in each route file to accomplish that. It would also help with ergonomics in general.

This is sort of the like the plugin functionality that bevy uses in it's builder.

Your Environment

• Rust Version (I.e, output of rustc -V): 1.69.0
• Actix Web Version: 4.3.1

[robjtede]

Duplicate or related to: #780 #1005 #1156 #2039 #2073 #2082 #2301

[robjtede]

Here's how to return App from a fn with v4.x:

actix-web/actix-web/src/app.rs

Line 698 in b1c85ba

fn can_be_returned_from_fn() {