microsoft: add azure active directory endpoint

Relevant Azure AD API documentation is here: https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-v2-protocols#endpoints

Fixes golang#257

Change-Id: Ia4fb136bbaa9d4445cd098f354070f16ace3b24b
Reviewed-on: https://go-review.googlesource.com/82315
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: rumyantseva

microsoft/microsoft.go

@@ -14,3 +14,14 @@ var LiveConnectEndpoint = oauth2.Endpoint{
 	AuthURL:  "https://login.live.com/oauth20_authorize.srf",
 	TokenURL: "https://login.live.com/oauth20_token.srf",
 }
+
+// AzureActiveDirectoryEndpoint returns a new oauth2.Endpoint for the given tenant at Azure Active Directory.
+//
+// For more information see:
+// https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-v2-protocols#endpoints
+func AzureActiveDirectoryEndpoint(tenant string) oauth2.Endpoint {
+	return oauth2.Endpoint{
+		AuthURL:  "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/authorize",
+		TokenURL: "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/token",
+	}
+}