Adopt C# example

C# SDK-style tree with a greet package and a stdlib-only solo package.

bazel run @rules_dx//:dx -- init //examples/adopt-csharp/...
bazel run @rules_dx//:dx -- generate //examples/adopt-csharp/...
bazel build //examples/adopt-csharp/...
bazel test //examples/adopt-csharp/...

Both tests pass (greet_test, pure_test).