Consume Things connect event in Bootstrap

[JMboya]

Is your feature request related to a problem? Please describe.

Currently, Bootstrap events.go handles removeEvent, updateChannelEvent, and disconnectEvent in the consumer package. However, we do not have a mechanism to consume a connectEvent when a Thing connect event

Describe the feature you are requesting, as well as the possible use case(s) for it.

Addition of a connectEvent struct in the consumer package. This struct would contain the thingID and channelID, similar to the disconnectEvent shown below. This event would be triggered whenever a Thing connects to the system.

Indicate the importance of this feature to you.

Must-have

Anything else?

Here's a rough idea of what the independent connectEvent struct might look like:

type connectEvent struct {
	thingID   string
	channelID string
}

Alternatively, define a connection event that consumes both Thing connect and Thing disconnect as below:

type connectionEvent struct {
	chanID  string
	thingID string
}

func decodeConnectionThing(event map[string]interface{}) connectionThingEvent {
	return connectionThingEvent{
		chanID:  read(event, "chan_id", ""),
		thingID: read(event, "thing_id", ""),
	}
func (es *eventHandler) Handle(ctx context.Context, event events.Event) error {
	msg, err := event.Encode()
	if err != nil {
		return err
	}
	switch msg["operation"] {
	case thingConnect:
		tce := decodeConnectionThing(msg)
		err = es.svc.ConnectThing(ctx, tce.chanID, tce.thingID)
	case thingDisconnect:
		tde := decodeConnectionThing(msg)
		err = es.svc.DisconnectThing(ctx, tde.chanID, tde.thingID)
	}
	if err != nil && err != errMetadataType {
		return err
	}

	return nil
}

This feature request is based on the current code in the consumer package:

[Dušan Borovčanin]

This is related to #1955.