Implement sorting that accounts for nulls

[Martoon-00]

Description

There is a TODO that hangs for quite a long time already.

Namely, in sorting, if some field allows null, it should be possible to specify whether these nulls should appear at the beginning or at the end. beam-postresql allows us to specify this, so SortingParams can allow that too.

A good place to start from: uncomment this code.

Acceptance criteria

• A concise syntax for API is provided. +date? vs +?date, something else?
• User can specify nulls sorting only for fields of Maybe X type, otherwise server rejects that.
• All necessary modules are updated among the following
  • Servant.Util.Sorting.*
  • Servant.Util.Dummy.Sorting
  • Servant.Util.Beam.Postgres.Sorting
• Parameter constructors for client handlers are added (in *.Sorting.Construction module).
  Probably a sane interface to aim at: nullsFirst asc #field or nullsLast desc #field.
• Tests are extended to cover the nulls sorting.