Added mutable getter for the openapi instance of a OpenApiRouter

[zannabianca1997]

There is currently no way of changing the OpenApi without breaking the router apart and recomposing it.

This enables modification of the API while mounting the routes.

I encountered this limitation while trying to build a router, and add a common security before nesting the router inside another. This will enable code like this:

let mut router = OpenApiRouter::default()
    .routes(routes!(handler1))
    .routes(routes!(handler2));

RequireUserToken.modify(router.get_openapi_mut());

let outer_router = OpenApiRouter::default()
    .nest("/inner", router)

[zannabianca1997]

This would also help with #1157. Most of the Router API is already accessible, this will enable access to the other half.

Another possibility would be to create a method that takes a type that implements Modify, but that would be a unnecessary restriction.

[zannabianca1997]

I opened an issue, #1263

This closes #1263

[juhaku]

Just missing a CHANGELOG.md entry from utoipa-axum/CHANGELOG.md. Add a new entry under Unreleased section (create if does not exists) and then add sub title Changed if does not exist then add the a bullet with * title (https://PR)

[zannabianca1997]

Must have missed it! Thank you for your time

[juhaku]

Awesome 👍