Similar to {T264669} Content Translation should be able to use the Parsoid in Mediawiki core. Currently, CX uses Virtual Restbase service.
[x] Write a client to use Parsoid in MW Core, use it in CX html->wikitext workflow
[x] Provide a mechanism to choose the existing RestbaseClient or Direct parsoid client by configuration. By default, it should use the current restbase approach.
[x] Add tests
[] Switch to a new client in multiple stages to evaluate
** [x] Small and Medium Wikipedias: https://phabricator.wikimedia.org/T337922
** [x] All Wikipedias: https://phabricator.wikimedia.org/T339322
[] Followups:
** [] Remove VRS related code from CX: https://phabricator.wikimedia.org/T339228 (Patch: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/ContentTranslation/+/930794)
** [] Remove wgContentTranslationDefaultParsoidClient cleanup: https://gerrit.wikimedia.org/r/c/operations/mediawiki-config/+/930798