add exta fields to context tracking amqp payload

**timestamp**:
Currently, if one were to write a daemon to consume rez context tracking AMQP messages (to dump into ELK for eg), there is no way to know the original timestamp indicating the actual resolve time. I believe RabbitMQ does have a plugin available to add this info into the message; however, rez does also publish these payloads in a separate thread, so even the message publish time does not necessarily reflect the actual resolve time.

**command**:
At method we're finding that we can spot old packages being used, but we can't tell where from. Having rez able to include the command in the payload would be useful. Given that rez-env is often run as a subproc (for us anyway, in our render farm), it may be necessary to store the command for each ancestor proc also.

Start by tracing the context tracking AMQP payload publishing and the rez-env subprocess handling described in the issue. Determine how resolve time and command provenance are currently captured. Done means payloads include the original resolve timestamp and the command context, including ancestor processes if required.

Written by the indexing model from the issue text.