Fix OptionsPattern for ProxyOptions

[datvm]

(I am pushing a pull request soon for this issue)

Currently Action<ProxyOptions> configureOptions is never called because of wrong DI declaration:

            services.Configure(configureOptions);
            services.AddTransient<ProxyOptions>();

Basically, ProxyOptions is always created and never configured if we put configureOptions in.

Suggestion: change the code to:

            services.AddOptions<ProxyOptions>()
                .Configure(configureOptions);

And currently, only WebSocketProxyMiddleware is using it, the correct DI declaration should be:

        public WebSocketProxyMiddleware(
            RequestDelegate next,
            IOptionsMonitor<ProxyOptions> options,
            Uri destinationUri,
            ILogger<WebSocketProxyMiddleware> logger)
        {
            _next = next;
            _options = options.CurrentValue;
            _destinationUri = destinationUri;
            _logger = logger;
        }

[damianh]

Thanks for PR!