C++ plugin registration

[RMeli]

Implement C++ plugin registration with the MTA_REGISTER_CXX_PLUGIN macro.

The code in 1de850a was suggested by Kimi-2.7-Code during a code review and implemented using Opus 5, so that a throwing C++ plugin's exception now reaches the caller with its original type (something I originally overlooked).

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[Luthaf]

The code in 1de850a was suggested by Kimi-2.7-Code during a code review and implemented using Opus 5, so that a throwing C++ plugin's exception now reaches the caller with its original type (something I originally overlooked).

I don't think we need a custom status code for this, it works without one in the C++ API of metatensor. (but this could also be thanks to the different treatment of CallbackError in here vs metatensor, I'll triple check).

[EricBoittier]

LGTM

[Luthaf]

I did a bit of refactoring here while splitting the changes into two separate commits, mainly making extra parameters std::optional in load_model; and re-using the C macro for plugins inside the C++ one, to make sure we don't have to maintain both places.

Let me know if the changes are good for you, then we can merge!

[RMeli]

I can't self-approve, but your changes LGTM, thanks!