The 3rd party package version used by the go server
may be different from the one in the user's source code
and will cause plugin version mismatched. Hence, we
should never import any external packages except the
Fission or built-in packages.
This PR replaces all 3rd-party packages with built-in packages.
Minimal golang runtime.
Functions are built as Go plugins, and the plugin is uploaded to fission.
See `environments/go/README.md` for instructions and `examples/go` for a usage example.
For now, we're re-using the 'code' field of the function to store the binary plugin. With v2 environments, this will be stored separately as a binary package.