In #7233, @mateuszkasprowicz has introduced a neat developer feature where developers of meta-estimators can use the new tag visual_block_kind to select serial or parallel display, and has streamlined the extender pattern to ensure html display of meta-estimators.

This pattern would be nice to have already in scikit-base, so all dependent packages can use it.

As usually, the pattern to move the logic one layer up is:

1. copy-paste with minor modifications, to scikit-base, ensure tests are also included
2. after a deprecation period, which can be silent in this case of private features, remove in sktime. Perhaps at the 1.0.0 mark, so we need to set release manager notes.