Rewrite OAuth2 API

Author: ras0219-msft

Release/include/cpprest/details/http_constants.dat

@@ -168,6 +168,8 @@ DAT(bearer,                         "bearer")
 DAT(client_id,                      "client_id")
 DAT(client_secret,                  "client_secret")
 DAT(code,                           "code")
+DAT(password,                       "password")
+DAT(username,                       "username")
 DAT(expires_in,                     "expires_in")
 DAT(grant_type,                     "grant_type")
 DAT(redirect_uri,                   "redirect_uri")