Allow solid material `density` to depend on parameterized field

Dynamic topology optimization problems require the `density` to depend on a parameterized field. There is currently a workaround using `addCustomDomainIntegral`, but it might be nice for the material model interface to handle this capability.

Start by reviewing the existing addCustomDomainIntegral workaround and the material model interface to determine where parameterized density is handled. Define the expected behavior for dynamic topology optimization, then add coverage for density driven by a parameterized field. Done means the material model supports this use case without requiring the workaround.

Written by the indexing model from the issue text.