POC: verify etag with md5

This is a proof of concept for validating the etag of the selenium.jar file by calculating a local md5 hash and comparing it.

Even though the etag is currently and md5 hash, Google guarantees only that it is unique (in a hashing sense), not that it will necessarily be an md5 hash. https://developers.google.com/storage/docs/hashes-etags#_ETags

Adding this complexity may not be worth the potential breakage it can cause. I can add an option to ignore checksums so that this can be worked around if it fails.

Please lend me your thoughts below.