jsonnet_library to support ext_code param

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.

Start by locating the jsonnet_library rule definition and reviewing how its existing parameters and Bazel BUILD-file inputs are handled. Clarify the intended ext_code and BUILD_USER behavior, then consider the issue done when jsonnet_library targets can pass BUILD-file values into a library and reuse the resulting value elsewhere.

Written by the indexing model from the issue text.