jsonnet_library to support ext_code param

[borg286]

I have a use case where I want to take values fetched from bazel BUILD files and pipe them into my jsonnet_library targets. More notably I want to pipe in the BUILD_USER into a value in my library, then use that library elsewhere. This is similar to the "data" param common in other bazel rules, but this one routes variables in BUILD files rather than the contents of files in.

[borg286]

I was able to work around it by having a jsonnet_to_json target where I used ext_code to pass the variable in. The jsonnet simply pulled the variable in via std.extVar and output it. I then used the "data" param on jsonnet_library to consume this json and pull out the values I wanted. Still it would be nice to avoid this hack.