Implement automatic schema collection for requests

[juhaku]

This PR implements functionality to recognize recursive schemas within types implementing ToSchema trait by (derive). These schema references will be collected to the single list of schemas which are added to the schema automatically from their usage point.

If schema is registered at schemas(MyType) for OpenApi then the type and it's schema references found recursively will be registered to the OpenApi.

If the type is defined in #[utoipa::path(request_body = ...)] the schema and it's recursively found schema references will be automatically added to the schema when the handler is registered to the OpenApi.

If the framework supports request body from http handler function arguments (like actix_web, axum) then the schema will be added to the OpenApi as well with recursively found schema references when the handler is registered to the OpenApi.

This commit also refactors the parsing for #[utoipa::path(request_body = ...)] Which enables the support for automatic schema reference recognition and collection. This also comes with the possibility to only add request_body(content_type = "my/type") without the schema if wanted. More over from now on the request_body supports defining multiple content objects allowing users to define multiple request body content's like the responses support.

Breaking changes

This request_body parsing is in over all breaking change and the changed spec need be checked from the docs or tests. In short the content_type = ... does not support anymore array syntax. E.g. content_type = [...] is not possible anymore. Instead to support multiple content types in request body on must define multiple content objects similar to responses.

 request_body(
    content(
        (Item = "content/type"),
        (Item = "content/type2"),
    )
 )

Example of supported syntax to demonstrate the automatic schema collecting from test_collect_schemas handler when registered to the OpenApi. Same way if Person was declared to OpenApi via schemas(Person) then Account would also get registered.

 #[derive(ToSchema)]
 struct Account {
     id: i32,
 }

 #[derive(ToSchema)]
 struct Person {
     name: String,
     account: Account,
 }

 #[utoipa::path(
     post,
     request_body = Person,
     path = "/test-collect-schemas",
     responses(
         (status = 200, description = "success response")
     ),
 )]
 async fn test_collect_schemas(_body: Person) -> &'static str {
     ""
 }

 #[derive(OpenApi)]
 #[openapi(paths(test_collect_schemas))]
 struct ApiDoc;

Fixes #692

Relates #662 #591 #965 #779