Add ability to filter the search results by a specific user

[beromir]

This PR adds the ability to filter the search results by a specific user.
To do this, you must pass a user as the second argument to the search method.

->search($searchString, Auth::user())

You can specify the user column name in the model. Otherwise, user_id is used as the default column name.

public string $userColumn = 'custom_user_column';

[freekmurze]

Instead of making it user specific, let's make it more general. I'd accept a PR that adds a second parameter to search that accepts a callable to modify the query.

Also make sure to update the tests and add an example to the readme.