Discussion about how to handle topology updates with Maintenance state/status

[italovalcy]

As discussed on this PR review, we should better discuss how the Maintenance of a node/port/link will be handled by SDX-Controller during topology updates. Creating this issue to unblock the PR from proceed and discuss this in more detail here.

[YufengXin]

1. Who? sends a "Maintenance" notice? an OXP or SDX?

2. What? a Link/port or Connections
   2.1 Link/Port in maintenance state/status by an OXP -> similar to link/port failure handling workflow except no re-provisioning is made.
   2.2 Individual connections: SDX-controller needs to add a new API end point: /connection/id/state
   -> in this case, AA integration is needed since only administrator is allowed to do so.
   2.3 Email users

3. OFF: when maintenance is off, same workflow options.