Support http proxies

[baywet]

Customers must be able to configure an HTTP proxy for the request adapter so the applications they build can run in networks that require accessing microsoft graph through a proxy.
Docs should also be updated once this is implemented

Tasks

• Implement support for proxy authentication
• Update public docs with information about proxy authentication #127

[baywet]

this should probably live in kiota http if any code change is required, related microsoft/kiota#371

[rkodev]

@baywet We might need to introduce a method on the abstractions.RequestAdapter known as UsingProxie(proxy string) so as the users can set this on the adapter. i.e

One approach can be

adapter, err := NewNetHttpRequestAdapter(authProvider)
adapter.UsingProxy("")
// or 
adapter.WithProxie("")

[baywet]

is this something that is set on the net/http client or on the request?

[rkodev]

This should be in the client itself

//creating the proxyURL
	proxyStr := "http://localhost:7000"
	proxyURL, err := url.Parse(proxyStr)
	if err != nil {
		log.Println(err)
	}

	//creating the URL to be loaded through the proxy
	urlStr := "http://httpbin.org/get"
	url, err := url.Parse(urlStr)
	if err != nil {
		log.Println(err)
	}

	//adding the proxy settings to the Transport object
	transport := &http.Transport{
		Proxy: http.ProxyURL(proxyURL),
	}

	//adding the Transport object to the http Client
	client := &http.Client{
		Transport: transport,
	}

[baywet]

it's a lot of boiler plate code, and we're already using the transport for our middleware. Could we just improve the http client factory to take into account the proxy settings? like GetDefaultClientWithProxySettings
Also how do you set proxy authentication (when needed) ?

[rkodev]

Added 2 factories to allow for proxy communication with and without authentication

// GetDefaultClientWithProxySettings creates a new default net/http client with a proxy url and default middleware
func GetDefaultClientWithProxySettings(proxyUrlStr string) *nethttp.Client {
	client := getDefaultClientWithoutMiddleware()
	client.Transport = getTransportWithProxy(proxyUrlStr, nil)
	return client
}

// GetDefaultClientWithAuthenticatedProxySettings creates a new default net/http client with a proxy url and default middleware
func GetDefaultClientWithAuthenticatedProxySettings(proxyUrlStr string, username string, password string) *nethttp.Client {
	client := getDefaultClientWithoutMiddleware()

	user := url.UserPassword(username, password)
	client.Transport = getTransportWithProxy(proxyUrlStr, user)
	return client
}

func getTransportWithProxy(proxyUrlStr string, user *url.Userinfo) nethttp.RoundTripper {
	proxyURL, err := url.Parse(proxyUrlStr)
	if err != nil {
		log.Println(err)
	}

	if user != nil {
		proxyURL.User = user
	}

	transport := &nethttp.Transport{
		Proxy: nethttp.ProxyURL(proxyURL),
	}

	middlewares := GetDefaultMiddlewares()

	return NewCustomTransportWithParentTransport(transport, middlewares...)
}