Support `STORAGE_EMULATOR_HOST` in the `google-cloud-storage` Gem

The Python version of this library lets you set `STORAGE_EMULATOR_HOST` which is very helpful for using this library in CI.

You can see it here.

https://github.com/googleapis/google-cloud-python/blob/94c09fdb034fa8fb0ca10778aaacdde9aa047d2a/packages/google-cloud-storage/google/cloud/storage/_helpers.py#L49-L50

**Is your feature request related to a problem? Please describe.**
Using this Ruby gem in CI is difficult because I have to manually mock out the storage instead of having it built into the gem.

**Describe the solution you'd like**
Supporty `STORAGE_EMULATOR_HOST` like you use in Python (here https://github.com/googleapis/google-cloud-python/pull/9219).

**Describe alternatives you've considered**
I currently manually handle this in my Ruby apps, but it's not my favorite thing to do.