Enhance OpenApi nesting with tags support

[juhaku]

This commits enhances the OpenApi nesting support by adding possibility to define tags [...] for the nested OpenApi endpoints. By default the fully qualified path to the nested OpenApi will become the tag for the endpoints if provided.

This commit also makes it necessary to define the argument names within the nest tuple to be constant across the utoipa macros and more readable. Path and api arguments are necessary.

Supported nesting syntax:

(path = "/path/to/nest/api", api = path::to:NestableApi, tags = [...])

Breaking! This is breaking change as this changes the utoipa::Path trait syntax to following. Prior this commit the path_item function took Option<&str> parameter to define default tag for the path operation but this is no longer necessary.

trait Path {
    fn path() -> String;
    fn path_item() -> PathItem;
}

Follow up PR for #930

Resolves #445 Resolves #872