[C++] Out-of-the-box support for Eigen & OpenCV types

[Wumpf]

We should provide a header with zero-copy conversion definitions.

Let's wait for

• #3794 to finish first though

[emilk]

The simple way of doing this is to add <rerun/opencv.hpp> and <rerun/eigen.hpp> with adaptors, all inlined

[grtlr]

This just came up in the community and I don't think this is blocked anymore?