Move version out of SDK. For now it's a copy rather than move: the part not addressed by this change is sdk/helper/useragent.String, which we'll want to remove in favour of PluginString. That will have to wait until we've removed uses of useragent.String from all builtins.
* Add useragent package
This helper provides a consistent user-agent header for Vault, taking into account different versions.
* Add user-agent headers to spanner and gcs
