[api] New, add custom filters

[dpgaspar]

Custom filters already exist on FAB and are widely used for base filtering on ModelViews or ModelRestApi. They allow you to implement a custom query that is always applied to all CRUD operations (normally to implement custom security for data access)

This PR implements a simple solution for adding custom filters to search filters, these custom filters can be defined and used the following way:

class FilterFullName(BaseFilter):
    name = _("Full name")
    arg_name = "first_or_last"

    def apply(self, query, value):
        return query.filter(
            or_(
                Contact.first_name.ilike(value + "%"),
                Contact.last_name.ilike(value + "%")
            )
        )


class SomeRestApi(ModelRestApi):
    ...
    search_filters = {"title": [FilterFullName]}
    ...