Add check to ensure BC breaking changes in rest descriptors are caught

[bshaffer]

This change in the Batch library was not caught by our Breaking Change detector because it's a configuration change rather than a PHP surface change.

I see two solutions to this:

1. Make the GAPIC generator create a surface artifact for rest configuration. Removal of these surface changes would be caught by the breaking change detector. This is similar to what we have in the Connection classes, where each method is implemented in a Connection/Rest class. The downside to this is it would create a LOT of overhead and also require a significant amount of work/changes in the generator, which would result in a lot of churn.

2. Write a "Configuration Breaking Change Detector" that runs alongside the existing "Surface Breaking Change Detector". This could be expanded to check breaking changes in any descriptor config files.