Add the option of using the external package msolve for computing the variety of a polynomial system. Only systems over ℚ are supported at the moment, because I cannot make sense of msolve's output in the positive characteristic case.

Note that this ticket does not add msolve as a dependency—see #31664 for that.

Upstream: Reported upstream. Developers acknowledge bug.

Component: commutative algebra

Author: Marc Mezzarobba

Branch/Commit: 0312ac6

Reviewer: Sébastien Labbé, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33734