Interface to avoid exposure of solver internals in usage of `get_node_variable`

Using `get_node_variable` to save extra node variables in the `SaveSolutionCallback` currently requires implementing an iteration over nodal values and/or directly accessing the containers:

- https://github.com/trixi-framework/Trixi.jl/blob/main/examples/p4est_2d_dgsem/elixir_navierstokes_vortex_street.jl
- https://github.com/trixi-framework/Trixi.jl/blob/main/examples/t8code_3d_dgsem/elixir_euler_ec.jl
- https://github.com/trixi-framework/Trixi.jl/blob/main/examples/tree_2d_dgsem/elixir_euler_blast_wave_amr.jl
- https://github.com/trixi-framework/Trixi.jl/blob/main/examples/tree_2d_dgsem/elixir_euler_vortex_amr.jl

[As suggested here](https://github.com/trixi-framework/Trixi.jl/pull/2874#discussion_r3007728760) by @DanielDoehring, this could be streamlined so that only the pointwise transformation of solution variables (and possibly gradients, if available in the parabolic cache) needs to be implemented. Possibly related: #2854.