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

[This change](https://github.com/googleapis/google-cloud-php/pull/6208/files/b1e0b806cbdb306b04112fb5b9d2807919078b94#diff-95631dd4150d2848b456932a4cc3dc212e7019bf847dad4eca37e60139ba9887) 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.