[c++] Consider taking std::string_view or similar for strings

[Wumpf]

Right now the C++ api uses nullterminated const char* everywhere which is nice for C (and ultimiately Rust) interop.
However, passing in std::string can get a bit clumsy:

rec.log(
        (std::string("world/") + name + "_pt").c_str(),
        rerun::Points3D({pos}).with_colors(color)
    );

The modern way is to use std::string_view but the problem is that they are not guaranteed to be nullterminated, complicating the FFI - either C needs to get more complex string types as well or we do a temp allocation for this!

[emilk]

Adding a size_t string_len argument to the C API should be fairly straight-forward