Add "force" argument to FeaturesAssignerInterface::assignConfigPackages()

Problem/Motivation

FeaturesManagerInterface::assignConfigPackage() includes a $force argument:

   * @param bool $force
   *   If TRUE, assign config even if already assigned to a package.

However, methods that invoke ::assignConfigPackage() lack the $force argument. This will be needed for #2595245: "excluded" configuration should not be automatically packaged, where we need to force assignment from the feature edit form.

Proposed resolution

Add $force argument to FeaturesAssignerInterface::assignConfigPackages() and related code as needed to pass to FeaturesManagerInterface::assignConfigPackage(.

Remaining tasks

API changes