Expose the negotiated HTTP version to request handlers #113

What I'm trying to do

Read, inside a request handler, the application protocol negotiated for the
connection (HTTP/1.1 vs HTTP/2, etc.).

The gap

NIOHTTPServer.ConnectionContext already carries it as a public let httpVersion: HTTPVersion, but a handler can't reach it: the only thing it receives is a RequestContext, whose connectionContext is internal. RequestContext surfaces the peer/local addresses (ConnectionInfo) and the validated peer certificate chain (PeerCertificate), but there's no capability for the negotiated version — so a handler has no way to tell which protocol it's serving.

Suggested shape

Expose it following the existing capability pattern, e.g.:

    extension HTTPServerCapability {
        public protocol NegotiatedHTTPVersion: RequestContext {
            var httpVersion: NIOHTTPServer.HTTPVersion { get }
        }
    }

    extension NIOHTTPServer.RequestContext: HTTPServerCapability.NegotiatedHTTPVersion {
        public var httpVersion: NIOHTTPServer.HTTPVersion {
            self.connectionContext.httpVersion
        }
    }

This keeps ConnectionContext internal to the handler surface and stays consistent with how ConnectionInfo / PeerCertificate are exposed. A minimal alternative would be just the public var httpVersion accessor. Happy to open a PR for whichever shape you prefer.