Define common search APIs

A generic search engine class should contain a set of methods to get the moodle components implementing global search, index documents in the search engine (current behaviour of search_get_iterators() for example)...

A class to act as a "document" representation would also be needed as currently there are hardcoded references to solr in modules implementing support for global search.

Even though these 2 classes should be abstract, not all their methods need to be abstract, I mean that for example there will be process that are common in all search engines (like gathering all iterators) but most of the processes should be defined by the specific search engine implementations; an example, the document class can define a abstract function __construct(); but the solr extension will be the responsible of extending it with a *function __construct()

{ $this->solrdocument = new SolrInputDocument();}

* same with search engine methods.