Add a solver-selection attribute to `SceneAPI`

[andrewkaufman]

Is this a new feature, an improvement, or a change to existing functionality?

New Feature

Feature Description

When a single Scene is configured for multiple solvers, it may have several FooSceneAPI applied. In this case we should offer a newton:solver attribute to indicate the "current preferred solver" for this scene.

Additional context or alternatives

Workarounds for this are to keep solver APIs on separate scenes & enable/disable them as needed, possibly driven by variants. If we don't add the attribute we should at least document the workarounds.

Code of Conduct

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• I have searched the open issues and have found no duplicates