This changes the User-Agent supplied by goose from being a generic "Go http package" to stating it is goose and what version.
Along with that change, it adds a goose.Version to the package, and goose.VersionNumber.
I didn't try to do the complex work that juju-core/version/Version does. I didn't need a lot of version number parsing, so it made more sense to enter the version as integers, and have it build the string trivially.