cpp codegen redesign, unit test support

cpp codegen iteration 2, based on discussions with three internal teams
that use c++. Refer to the design doc "aconfig c++ codegen" for detailed
design. At a high level, we generate two sets of code artifacts with the
same signatured api: one for production that without any local flag
override capability, one for unit test that allows local flag overrides.
It supports static methods style interface as well as injection pattern.

Refer to the test points in the codegen_cpp.rs for examples of generated
code.

for production target codegen: aconfig create-cpp-lib --cache <cache> --out <out dir>
for test target codegen: aconfig create-cpp-lib --cache <cache> --out
<out dir> --mode test

Bug: b/279483801
Test: atest aconfig.test
Change-Id: I92fefb9623d5435525339a74f57bbd36d0afef08