Support building yt/cpp and yt/yt/core with vanilla protobuf

After this PR yt/cpp and yt/yt/core are possible to be built both with Arcadia protobuf (that uses TString as a string) and vanilla protobuf (that uses std::string as a string). To achieve so, a couple of interoperability primitives are introduced.

* `TProtobufString` is an alias to protobuf string type, i.e. it can be `TString` or `std::string` depending on the protobuf implementation.
* `IsVanillaProtobuf` and `IsArcadiaProtobuf` are the constexpr boolean values that allow to check protobuf implementation both in the compile time and runtime.

The most challenging interoperability issue solved here is a string copy between protobuf message and C++ code that has a form of `TString str = msg.str()`. This code works perfect with Arcadia protobuf but does not work with vanilla protobuf. To solve it, a previously introduced primitive `FromProto<TString>` is used. This expression makes the most efficient cast possible between protobuf string and C++ string. Internally, it is just a copy in both cases. Since TString is CoW by default, this expression is almost zero-cost (actually it's just one atomic operation), so no degradation is expected for YTsaurus server builds. The most hot code is handled differently to avoid even atomic operations (see `GetRequestTargetYPath`). In case of vanilla protobuf string is copied, however there are no places in C++ SDK where it might be a problem. If such issues would appear, performance-critial code can be rewritten in `GetRequestTargetYPath`-style.

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1a6f3e02cb6e83915102c24b73bc8734f6a48e74

Pull Request resolved: https://github.com/ytsaurus/ytsaurus/pull/466

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