feature(events): add columns about properties

[terry-xiaoyu]

Add columns about MQTT5.0 Properties in all of the events. The colomns
added this time:

• pub_props for message.publish, message.dropped and message.delivered

• conn_props for client.connected

• disconn_props for client.disconnected

• sub_props for session.subscribed

• unsub_props for session.unsubscribed

• pub_props and puback_props for message.acked

Note that the column headers in message.publish will be removed
in the next major release, as we'd better put all the available columns
at the top-level of the sql selection result.

[terry-xiaoyu]

Add a new column metadata, we will put all rule-related data in it:

• metadata :: #{rule_id := binary(),
  action_id := binary(),
  atom() => binary() | map()
  }

[terry-xiaoyu]

Fix a bug with regards to FOREACH:

FOREACH clause should always returns a list of map.

That is, given the following SQL:

FOREACH payload.devices
FROM "t/#"

And an input:

{"payload": {"devices": [1, 2]}}

The output should be:

[#{item => 1}, #{item => 2}]

And should NOT be:

[1, 2]