Add methods to manage enforced dependencies

In #2224581: Delete forum data on uninstall we're adding the ability to enforce configuration entity dependencies. This issue will add methods to add and remove enforced dependencies. It will also remove ConfigTest::setEnforcedDependencies and use the new methods instead. The documentation on the method that adds an enforced dependency must outline how the use-case is different to the addDependency method and when it should be used.

Since #2224581: Delete forum data on uninstall is yet to land this issue is postponed.