Mattermost-server: [Help Wanted] [MM-9990] Add support to REST API for sending ephemeral messages to users

[esethna]

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Notes: Jira ticket

It would be useful for bots and integrations to be able to send ephemeral messages to users by using the REST API.

For this ticket, you'll need to do a few things:

1. Add a new permission controlling who can send ephemeral messages to a user and give it to system admins
2. Modify the POST /api/v4/posts endpoint to accept system_ephemeral as a post type, check the new permission and send the ephemeral post (instead of going through the regular post creation flow)
3. Add API documentation detailing the changes to https://github.com/mattermost/mattermost-api-reference

If you have any questions, please ask Joram (@joram on pre-release and @jwilander on GitHub).

[DSchalla]

As discussed with @jwilander in the Contributors chat I'll take a shot at the issue and keep you posted on the progress here.

[DSchalla]

Update: Majority of the implementation is working, it was implemented in a own API handler since there was no nice way to merge it in the current POST /posts handler. I am fixing last issues and then open a PR for review, additional PRs to the webapp and api docs repositories will be required.

[DSchalla]

I think this issue can be closed after the merge, unless there are any questions left.
Thanks again for the guidance!

[jwilander]

Fixed by #8611