Support partial matches

Attached is a patch, against 2.x with the solution described above.

Oops, faulty patch in #2.

Here's a fresh one.

Third time is the charm!

Hello,
Thanks for this job, its usefull ... but :

The patch applies well but it does not work with "Contains" option.

In search_api_opensearch/src/SearchAPI/Query/SearchParamBuilder.php, it should be :

      $params['query_string']['query'] = match($matching_mode) {
        'contains' => '*' . (string) $luceneQuery . '*',
        'starts_with' => (string) $luceneQuery . '*',
        default => (string) $luceneQuery,
      };

Instead of :

      $params['query_string']['query'] = match($matching_mode) {
        'partial' => '*' . (string) $luceneQuery . '*',
        'starts_with' => (string) $luceneQuery . '*',
        default => (string) $luceneQuery,
      };

Or change the option value in .module file in select list "$form['options']['opensearch']['partial_match_mode']"

I tested this locally with '*' . $luceneQuery . '*', for the searching. It works but behaves sometimes very strange when it comes to the results out of the search index.

When the query is longer then 10 characters a lot of entries are in the results.
I debugged that with explain and saw that all the results had a `field_exist` constant score. This is only happening with the above changes and not without. Currently unsure why this happens and how this could be avioded.

I'm also unsure how this influences the Fuzzy search logic which is set to AUTO. Perhaps this needs to be disabled for a real "contains" to work?

I was able to confirm that it works with khaled.zaidan's #5 patch.
However, I think it would be better if the notation in the select box and the value matched.