New `proxy` attribute: `unexpected_status`

[johakoch]

Implement a new attribute for the proxy block: unexpected_status.

Use case: see discussion https://github.com/coupergateway/couper/discussions/723.

Especially if the proxy block is within wildcard endpoint block

endpoint "/**" {
  proxy {
    backend = "api_backend"
  }
}

all sorts of valid status codes can occur due to the endpoint proxying to potentially a lot of backend API endpoints.

So if you want to react to a specific error status code, e.g. 401, with an error_handler "unexpected_status" and proxy all other responses, you cannot use the expected_status attribute. Here, an unexpected_status attribute would come in handy:

endpoint "/**" {
  proxy {
    backend = "api_backend"
    unexpected_status = [401]
  }

  error_handler "unexpected_status" {
    # try to get a fresh token
  }
}

Should the new attribute be mutually exclusive with expected_status?

I don't think that the other block types that have expected_status (request, beta_token_request, health) would profit from an unexpected_status attribute. All three are usually used for specific requests, where only a very limited set of status codes are treated as valid.