Re-expose customEndpoint configuration option #1315

Is your feature request related to a problem? Please describe.

I am trying to write tests for code which uses GCS. these tests are run on a CI platform. Having the tests run against a real bucket means potential erratic failures because of network issues and possibly billing for "useless" operations. I am trying to use a locally deployable emulator (even if it doesn't support all the features) but that proves harder than it should be.

In #18 (comment) @stephenplusplus suggested to use

const Storage = require('@google-cloud/storage')
const gcs = new Storage({ customEndpoint: true })

but this feature no longer works, even though the feature is still available in the @googlecloud/common library

Describe the solution you'd like

I would like the at least that parameter to be restored in StorageOptions and propagated to the GoogleAuth client. I am not sure if StorageOption should add more options from MakeAuthenticatedRequestFactoryConfig

The customEndpoint parameter then needs to propagated when building the config object which is passed to the Service abstract class in the Storage class constructor

Describe alternatives you've considered

My current workaround is to use the apiEndpoint to run against a local store but provide valid credentials to a free firebase project in order to pass through auth. This means our test suite cannot work without access to google's servers and the CI plaform uselessly sends requests to google auth servers.

I am willing to contribute a PR if it can help.