Automatically add to account_data "m.direct" direct rooms members

To create a direct message room we need to call

client.createRoom({ 
   is_direct: true, 
   invite: [userId], 
   visibility: "private" 
})

This creates the room, but as per the matrix spec:

Both the inviting client and the invitee’s client should record the fact that the room is a direct chat by storing an m.direct event in the account data using /user/<user_id>/account_data/.

As far as I can tell, this isn't done automatically by matrix-js-sdk.

Manual management is impossible for servers that configure auto-joining on invites. Without m.direct update, it's impossible to tell which rooms are DM rooms. This might mean that this is a Bug report rather than a Feature request.

Describe the solution you'd like

When a user joins a DM room (one that was created with the above options), matrix-js-sdk should make sure the m.direct account data is correctly set as per the matrix docs.

Start by tracing the client.createRoom entry point and the room-join path, then review the m.direct account-data endpoints described in the issue. Done means direct rooms created with the shown options have the correct m.direct account data after the user joins.