* feat: include connection id in the plugin context
* refactor: group ConnectionId next to SessionId in plugin Context
Addresses review feedback to keep related identifier fields adjacent.
* fix(files): forward Connection-Id on file uploads to plugin hooks
The webapp uploadFile XHR didn't attach the Connection-Id header, so
FileWillBeUploaded plugin hooks always received an empty ConnectionId.
Read it from the websocket selector and set it on the request, matching
how drafts and channel bookmarks already do it. Adds a server-side test
asserting the connection id propagates through pluginContext.
* fix(lint): reorder file_actions imports to satisfy import/order
* Document ConnectionId on request.Context
This originated from https://github.com/mattermost/mattermost/issues/15249.
However, the original idea was discarded https://github.com/mattermost/mattermost/issues/15249#issuecomment-709713065
as being too complicated to implement. Then I had another
idea to implement it just for session objects.
My thinking was that since every single request allocates a new
session struct, it would be good to use a sync.Pool for that.
However, 4 years later, now we know that the primary bottleneck
in app performance comes from websocket event marshalling.
Therefore, while it would be good to do this, it is difficult
to do it correctly (as shown by the numerous racy tests).
Hence, reverting this.
```release-note
NONE
```