Add clean way for retrieving an index's data alterations and processors

Problem/Motivation

I've built a datasource that needs the options of a related processor to work correctly. Unfortunately it's not possible to access the defined processors from outside of an index.
Currently this is only possible by "hijacking" the method:

class SearchApiIndex Hijack extends SearchApiIndex {
  public static function getProtectedProcessors(SearchApiIndex $index) {
    return $index->getProcessors();
  }
}

Proposed resolution

Change visibility of SearchApiIndex::getProcessors() from protected to public.

Remaining tasks

Make a decision :)

User interface changes

none

API changes

SearchApiIndex::getProcessors() will become more visible. Shouldn't have any impact on existing code.