Mattermost-server: Expand Plugin API to trigger user typing event

[mattermod]

Other systems such as MS Teams, it provides context to the user on longer running operations that the bot is “thinking”. See example below of the MS Teams Github Bot “typing”

1. Expose a REST API for /users/ https://github.com/mattermost/mattermost-server/blob/master/api4/user.go#L22 to let System Admins trigger the typing event for any arbitrary user. Follow https://github.com/mattermost/mattermost-server/blob/master/api4/plugin.go#L36 /plugins/{plugin_id}/enable example on how to check for system admin permissions and/or parse post params. For regular users, the api will trigger the typing event for the currently logged in user.
2. Expose same typing API to PluginsAPI https://github.com/mattermost/mattermost-server/blob/master/plugin/api.go#L16

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JIRA: https://mattermost.atlassian.net/browse/MM-18756

[mdabydeen]

I would like to take a look at this one

[ruzaikr]

@mdabydeen are you still working on this? If not, I'd love to take it up :)

[mdabydeen]

Hey @ruzaikr I am still working on this

[lieut-data]

@mdabydeen, how's your efforts on this ticket? Anything we can provide guidance on?

[hanzei]

Making this available for the public again

[attilamolnar]

Hey, I would like to take a look at this

[lieut-data]

Thanks, @attilamolnar!

[attilamolnar]

I'll cook something up in an hour or so

[attilamolnar]

Diffs:
Fix a small oversight in api4/user_test, needed for an upcoming test
https://github.com/mattermost/mattermost-server/compare/master...attilamolnar:fix-assert-ws

First take at the feature itself
https://github.com/attilamolnar/mattermost-server/compare/fix-assert-ws...attilamolnar:MM-18756_typing-api

Let me know how you want to proceed. I would appreciate some feedback before updating the API docs and such.

[lieut-data]

@attilamolnar, can you submit a draft PR linking this issue for review? That will be an easier mechanism to give early feedback on the approach.