[feature] Server WFS3 api sorting

[elpaso]

For documentation: the API is really self-documented thanks to OPENAPI, but here is some information about the new parameters:

 "sortby" : {
    "name" : "sortby",
    "in" : "path",
    "description" : "Sort results by the specified field name",
    "required" : false,
    "schema" : {
      "type" : "string"
    }
  },
  "sortdesc" : {
    "name" : "sortdesc",
    "in" : "path",
    "description" : "Sort results in descending order, field name must be specified with 'sortby' parameter",
    "required" : false,
    "schema" : {
      "type" : "bool"
    }
  },

[qgis-bot]

@elpaso
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[pblottiere]

It looks good to me.

Thanks @elpaso 👍

[qgis-bot]

@elpaso
A documentation ticket has been opened at qgis/QGIS-Documentation#5764
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!