Refactor BrowseRequest and SearchRequest to use a common base object

[as-smets]

I want to have a BaseRequest object, containing common fields of BrowseRequest and SearchRequest.
For the moments the duplicated fields we use are : offset, resultsPerPage, facets, preFilterExpression, sortBy, sortAscending
And this list can only grow
I checked on other classes, it's used for example with QuizRequestBase