SQL feature provider: add nullOrder option

[cportele]

This change adds a query generation option nullOrder to SQL feature providers to control the sort order for columns with null values. If no value is provided, the default order of the database applies. For last, the rows with null always are last for both ascending and descending. For first, the rows with null are always first. This is implemented by adding NULLS LAST or NULLS FIRST to each sort field.

Example:

queryGeneration: 
  nullOrder: last

[cportele]

@azahnen

This should be ready for review.

If configured, the order is specified for all sort fields including the sort id field (which never should be null, so for these cases the addition has no effect).

I tested it with the "vineyards" API, by adding the option and by adding "village_info" to the sortables.

[azahnen]

Works as expected.