Storage endpoint as env var

[tarsisazevedo]

Is your feature request related to a problem? Please describe.
In order to test storage integration locally I need to setup host in StorageOption Builder by myself, like this:

final storageEndpoint = System.getenv("STORAGE_ENDPOINT");
StorageOption.newBuilder().setHost(storageEndpoint).build().getService();

Describe the solution you'd like
It can be easier and more time proof if StorageOption.DEFAULT_HOST use a env var to get it's value.

Describe alternatives you've considered
I think about something like this in StorageOptions class:

private static final String DEFAULT_HOST = System.getenv().getOrDefault("STORAGE_HOST", "https://storage.googleapis.com");

I can open a PR if you agree with this implementation or have any other idea.

[BenWhitehead]

@frankyn Do you know if anything similar to this is done for any of the other clients?

[frankyn]

We have it in the Node.js, Go, and Python storage clients, but it's not necessarily "official".

It was started a side project for performance benchmarking.

Maybe it's time to help make these official across libraries at this point. There are quirks that need to be ironed out a bit more though.

[tritone]

This has definitely been a source of confusion and bugs in the Go library, so it has to be done carefully. I do think it's a good idea to be consistent about env var naming and how this is supported across libraries though