LPS-40873 Add initiated timestamp to WebRTCConnection

I think `initiatedTimestampMs` explains the purpose of this attribute.
According to Wikipedia, a timestamp is:

  a sequence of characters or encoded information identifying when a
  certain event occurred, usually giving date and time of day,
  sometimes accurate to a small fraction of a second.

The `Ms` suffix is added to specify that it's a timestamp in
milliseconds since Unix timestamps are most commonly in seconds.