Improve http_status validation

[kurochan]

Currently, the validation of the http status input is a numeric range, so an invalid status code can be entered. Change to check if the http status code exists or not.
related to #1389 .

Even if this validation were added, it is still possible to set it from the Web UI( https://one.dash.cloudflare.com/ ), so that needs to be modified as well.

Those that exist as http status but are not normally used can be registered, but I think this should be handled by the web UI.
(Personally, I don't see a problem with using the joke status code because it's funny.)

[lazysegtree]

@kurochan, this apparently creates a dependency about http status codes on the go source code.

• This creates a hard dependency on update of go codebase for any new HTTP status code among the unassigned http status code (https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml). Only the status code that are mentioned in https://github.com/golang/go/blob/master/src/net/http/status.go would stay valid.

Also there are some "Unofficial codes" that might be a valid use case (See https://en.wikipedia.org/wiki/List_of_HTTP_status_codes), for which go's StatusText function would not return any text.

• For example HTTP 218, 509, 529, and many more

(Here is a screenshot from wiki page about the additional unofficial codes, used by cloudflare too)