Add support for dynamic container of local solvers

This allows to have different local solvers in `NonlinearGSStep`.
The feature existed before but was restricted to statically sized
containers. This patch adds support for dynamically sized ones.
4 jobs for feature/nonlinear-gs-dynamic-localsolver in 9 minutes and 47 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #177679

00:01:18

passed dune:git clang C++17 with examples #177681

00:06:35

passed dune:git gcc-8 C++17 #177680

00:02:00

passed dune:git gcc-8 C++17 with examples #177682

00:09:44