This adds a boolean flag use_auth_w_custom_endpoint - whether authentication is required with custom endpoints

• only used when a custom endpoint in set in conjunction
• defaults to True and uses authentication
• if set to False, bypasses authentication

For backwards compatibility

• by default, auth is bypassed when STORAGE_EMULATOR_HOST is set
• add bool _is_emulator_set to improve readability

Refactored to ensure preprod testing is not adversely impacted

• updated docstring to note internal testing used environment variables

Fixes #895 and internal b/258502260