[feature] Add repository rule for translation of "properties" file into a .bzl

[eustas]

Context: bazelbuild/bazel#18617; example: knik0/faad2#162

Just asking if it is appropriate. If so, I'll prepare a PR.

Reasoning: normally to translate "properties" into skylark variables we have to

1. add repository rule into e.g. workspace.bzl
2. load and use that rule in WORKSPACE
3. load resulting .bzl in target BUILD file

Step (1) could be removed if such a rule is a part of bazel-skylib. As a stretch goal, we could add more input formats alongside nearly-native JSON.

[tetromino]

I'm not sure if this would be best done as a repository rule or a module extension. (@Wyverald, are repository rules going away?)

[Wyverald]

Repo rules are not going away. This seems appropriate as a repo rule -- pass a JSON file in, spit a .bzl file out.

[tetromino]

Thanks! In this case, a PR is welcome