Migrate protobuf from vendored 3.4.1 to vcpkg 3.21.12 (last release before Abseil became mandatory). vcpkg protoc + libprotobuf; source fixes for the 3.4->3.21 API (cpp_helpers.h, duplicate ext registration, Status::ToString); remove the ~47MB vendored 3.4.1 tree. Both blade and bazel builds + proto/RPC tests pass; protobuf install is abseil-free.
* Stripped thirdparty/ prefix when including external headers.
* Introduced dedicated folder for different building systems.;
* Using canonical path for include protobuf's headers.
* Using canonical path for googletest.