Datanommer does not store the `agent_name`

[abompard]

The message's agent (in the agent_name property) is not store specifically by datanommer, only as part of the usernames. It would be interesting to filter on the agent name only, see fedora-infra/fedbadges#100

[abompard]

Obviously this will require a schema change, and it will increase the DB size (data + index).
This should be a column in the messages table, because there's only one agent_name, and joins to a hypertable are expensive in timescaledb.
The extract-users command should be updated to take it into account.
The agent and not_agent arguments should be added to the Message.grep()
Also, datagrepper should be updated to add the new filters