Description of the problem / feature request:

Change the file format used to configure C++ toolchain from text proto to Skylark

Feature requests: what underlying problem are you trying to solve with this feature?

• It will make maintenance of the C++ toolchain easier, it will allow getting rid of the duplication and boilerplate using standard programming practices, e.g. we will be able to provide a library of common features.
• We will benefit from the tooling being developed for Skylark (e.g. testing, debugger).
• It will make future C++ rules consistent with all the other Skylark rules. It will unblock rewriting of cc_toolchain rule in Skylark.

More info on this effort can be found in @hlopko 's doc: https://docs.google.com/document/d/1Nqf16jqDGWSrPp4VuRxh0iNnVBoAXsO0meDH69J9xoc/edit#heading=h.r30au8wdo4dh