Machine readable drenv logs

[nirs]

Current drenv logs are hard to process by a machine.

Instead of text based logs, use jsonlines format:

$ drenv start --json envs/regional-dr.yaml
{"time": "2024-03-31 11:24:09,869", "level": "DEBUG", "context": "main", "message": "Created new process group 105239"}
{"time": "2024-03-31 11:24:09,869", "level": "INFO", "context": "rdr", "message": "Starting environment"}

This allow the tool running drenv to process logs and display interesting events while
storing the logs to file.

It also allows easier processing of the logs later to compute stats.

The implementation is easy, using a json Formatter