Add `encoding` field to the wiki page entity

Merged Francisco Javier López requested to merge 336792-fj-add-encoding-field-to-wiki-api into master

What does this MR do and why?

In #336792 (closed) we want to start rendering wiki pages asynchronously. In order to do that, we're going to leverage the existing wikis API. Nevertheless, we need to accommodate it in order to keep some of the existing functionality working.

In this MR we're adding the encoding field to the wiki page entity This field will return the encoding of the wiki page content is written in.

How to set up and validate locally

  1. Get a wiki page using the existing endpoint. Ex: http://127.0.0.1:3000/api/v4/projects/6/wikis/home
  2. Check that the encoding field is present and correct

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #336792 (closed)

Edited by Francisco Javier López