Use a common rule for exes/tests and turn the public APIs into macros

[j3parker]

This relates to issue #71. We need to move these to macros because we need an extra cc_binary target for each DLL to generate a wrapper exe that will run the exe/DLL with the dotnet runtime. This part is a bit messy but easy so I'm doing it in it's own PR.

I'm worried about keeping the documentation up to date: https://github.com/Brightspace/rules_csharp/blob/master/docs/APIReference.md Starlark generates documentation differently for macros vs. rules (rule attributes have "static types" (e.g. label list) unlike macros. As a user of these rules I don't think we want that distinction for these rules. I think it will be easy to hack this because the way we generate docs we have the ability to do basically whatever. We could always generate fake docs-only rules (that don't do anything) that have the right args and docstrings to get Starlark to do what we want. I'm going to open up a separate issue for this because it's a lower priority for me and this change should be docs-neutral anyway.

As a bonus, we will now generate runtime.config.json files for tests which @jimevans noticed we were missing.

[j3parker]

🤦 I keep forgetting to open these PRs from a fork. Buildkite + required status checks kinda require that (unless I click the cheaty button). I'll reopen.

[j3parker]

I configured the branch protection to "Include Administrators" so git push won't work next time