Add api version constants #1065

Problem you are trying to solve:
Dynamically get latest version of lib available to avoid having to change namespace in our code

Example:

$version = Constants::API_LATEST_VERSION;
$clientClass = "Google\\Ads\\GoogleAds\\{$version}\\Services\\GoogleAdsServiceClient";

$client = new $clientClass();

Solution you'd like:
Having latest version in Contants.php

public const API_LATEST_VERSION = 'V18';

Additional context:
May be add oldest version too like
public const API_OLDEST_VERSION = 'V16';

Thx