Support inferring rate-limited endpoints in OpenAPI

Starting in .NET 7, it is possible to enable rate-limiting on certain endpoints. When this is the case, the endpoints might return a response with a 503 status code.

We don't currently pass this information to any OpenAPI annotations, but we should probably document this when it is possible to infer.

Note: this is similar to the problem with the global `ProblemDetails` service (pun intended) and is related to https://github.com/dotnet/aspnetcore/issues/44192.