Add constraint preserving boundary condition product for ScalarTensor

[guilara]

Proposed changes

We write a boundary condition for the ScalarTensor system that combines the Bjorhus BCs for the metric sector and the constraint-preserving BC of CurvedScalarWave.

This is not a true constraint-preserving boundary condition since the gh BCs assume no backreaction of the scalar, and conversely, the constraint-preserving boundary conditions of the scalar assume a fixed spacetime geometry. I find that this approximation behaves well during evolution.

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Further comments

[guilara]

@nilsdeppe Thanks for the review. Yes, I squashed them.