Allocations for `SummaryCallback` with custom integrators

We observe allocations for the custom integrators, see

https://github.com/trixi-framework/Trixi.jl/blob/2dfde7faf3cc74f066d86148ae6c99ed9e58fa79/test/test_structured_2d.jl#L762-L774

https://github.com/trixi-framework/Trixi.jl/blob/2dfde7faf3cc74f066d86148ae6c99ed9e58fa79/test/test_tree_2d_advection.jl#L204-L215

and in the current PR #1871

https://github.com/trixi-framework/Trixi.jl/pull/1871/files#diff-4e3f4096536ca3da518b2f02b6f9197bc83ed9a7aec477b329951667dc1f830c

I observe that the allocations seem to be caused by the `SummaryCallback`.

For instance, for the example [tree_1d_dgsem/elixir_hypdiff_nonperiodic.jl](https://github.com/trixi-framework/Trixi.jl/blob/main/examples/tree_1d_dgsem/elixir_hypdiff_nonperiodic.jl)

the alloc check

```julia
t = sol.t[end]
u_ode = sol.u[end]
du_ode = similar(u_ode)
@allocated Trixi.rhs!(du_ode, u_ode, semi, t) # 7520
```
gives `7520` (as for the PERK2 test).

If one supplies the callbacks

```julia
callbacks = CallbackSet(#summary_callback,
steady_state_callback,
analysis_callback, alive_callback,
save_solution,
stepsize_callback)
```

instead of

```julia
callbacks = CallbackSet(summary_callback,
steady_state_callback,
analysis_callback, alive_callback,
save_solution,
stepsize_callback)
```

one obtains `0`.

Start with the allocation checks in test/test_structured_2d.jl and test/test_tree_2d_advection.jl, then reproduce the example using examples/tree_1d_dgsem/elixir_hypdiff_nonperiodic.jl and compare CallbackSet configurations with and without SummaryCallback. Done means the callback remains enabled without the reported allocations in the Trixi.rhs! check.

Written by the indexing model from the issue text.