feat: add endpoint to set message flags via the OCS API

[steven-mpawulo]

Adds MessageApiController::setFlags (POST /message/{id}/flags) so external integrations using the OCS API can mark messages read/unread

Fixes #13363

[SebastianKrupinski]

Hi @steven-mpawulo

Any chance this can wait until this is merged... #12800

That will event give you the ability to bulk change flags.

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