Adopt C++ example

C++ tree with a greet package and a stdlib-only solo package.

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

Both tests pass (greet_test, pure_test).

copts reaches the compiler exactly as written, so -std=c++17 here stays C++17. Warnings are errors because the repository selects the treat_warnings_as_errors toolchain feature, not because the wrapper adds a flag. The same feature can also be selected on the command line:

bazel build --features=treat_warnings_as_errors //examples/adopt-cpp/...

See cc/rules/README.md for the wrapper contract.