register structured units with yaml serialization - #12492

This fixes the structured unit serialization.
Doing the same for a numpy.void will take a separate commit.

The following work

from astropy.io.misc.yaml import dump, load
import astropy.units as u
unit = u.Unit("(eV, eV, m)")
dump(unit)

arr = u.Quantity([(0, 0, 0.6), ], u.Unit("(eV, eV, m)"))
dump(arr)

This does not

arr = u.Quantity((0, 0, 0.6), u.Unit("(eV, eV, m)"))
dump(arr)

Fixes #12485

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