Add a new utility which will register an autoloader that will load either 'Current' (for a user specified version) or 'Latest' as a namespace path

[DeveloperRob]

Will fix #1065

Add a new utility which can register an autoloader that allows user code to specify \Google\Ads\GoogleAds\Latest to always use the latest API version, or Google\Ads\GoogleAds\Current to always use a specified version (allowing upgrades to newer versions by updating a single location in the code).

I would welcome any feedback or suggestions on improvements / changes. I am also aware that users using setGoogleAdsAutoloaderLatestVersion are running the risk of changing API version without reviewing the migration guide, however as each new release is a major version increase they should be manually updating their composer.json, and the original request seemed to want to be able to always use the latest version.

Finally, I have not added tests yet, as I would like to see if there is any feedback that requires changes first (although I have manually tested the code). I am happy to provide tests once the main code is in an acceptable state.

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