go-jsonnet: add jsonnetfmt binary (#91953)

Add binary jsonnetfmt to go-jsonnet package

go-jsonnet added a jsonnetfmt implementation in [0.16.0](https://github.com/google/jsonnet/releases/tag/v0.16.0)
As nix includes the C++ jsonnetfmt implementation in the main jsonnet package, it makes sense to mirror that pattern in the go package.