Improve extendability of SearchController and SearchActionFactory classes

Problem/Motivation

In our usecase it was necessary to extend the SearchAction, SearchActionFactory and SearchController classes.
Some improvements are required for the following problems:

• SearchActionFactory class variables have private visibility
• SearchActionFactory does not have an interface
• The $searchAction variable would be useful as a class property in the SearchController, this way we can use data from it in extended methods that don't have it as an argument (getSearchHeader() for example)

Proposed resolution

• Within SearchActionFactory, change the visibility of $size, $termStorage and getHierarchicalValues() to protected instead of private
• Implement a SearchActionFactoryInterface and replace type hints
• Add a $searchAction class property to the SearchController