Fetching posts should allow for sorting

[RiccardoM]

Is your feature request related to a problem? Please describe.
Currently when fetching posts using the REST API there is no way to sort the posts before the pagination. This results into the impossibility of fetching (e.g.) the latest posts sorted by descending creation date.

This is particularly useful when developing applications that needs to show the posts in a timeline.

Describe the solution you'd like
Improve the /posts REST API endpoint to accept a sorting parameter and a sorting order parameter. One example could be

/posts?sort=created&sort_order=descending

Describe alternatives you've considered

Additional context