Search API verify use of search_scope method

Consider renaming the search_scope method in the search controller to either: user_requested_scope or raw_scope_param so it's very clear what it is

The following discussion from !220846 (merged) should be addressed:

• @terrichu started a discussion: (+2 comments)

  question (non-blocking): could this get overridden by the backend? I'm actually not sure if the same thing happens in the API as in the SearchController and want to open a follow up to look into it. I know the SearchController prefers to use search_service.scope (through the search presenter)