Export helper for custom-run filters

[IlyaSemenov]

See #4 (comment):

it'd be helpful to export the helper that filters query against a filter object such as { field__in: [1,2] }.

Sample code:

query FilteredAssets($limit: Int, $offset: Int, $events_filter: Filter) {
  assets(limit: $limit, offset: $offset, filter: $filter, eventsFilter: $eventsFilter) {
    name
    # events could be included or could be omitted - that doesn't affect the assets query!
    events(filter: $eventsFilter) {
      id
      event_type
    }
  }
}

Resolver:

Assets: async (parent, args, context, info) => {
  const assets = await resolveGraph(
    context,
    info,
    Asset.query().whereExists(
      Assets.relatedQuery('events').where(
        // construct custom condition from args.eventsFilter
        // <------ TODO: use the newly exported function
      )
    )
  )
  return assets
}