Automatic reallocation of mjData buffer

[saran-t]

MuJoCo is designed to not dynamically allocate any memory at runtime, which is great for e.g. embedded applications. However, in many higher-level use cases, it is desirable to have a mechanism that would allow the mjData buffer to grow dynamically when full, rather than triggering a memory error.

In terms of implementation detail, this is possibly best done as a new C++ API that wraps the existing C API.