make the random log generator generate better logs

[aybabtme]

the genny command is useful to generate random log data for iterating on humanlog, but the current random log generation is not great:

• its only logfmt, and barely even
• the data isnt realistic at all
• it doesnt generate messages in other log formats that humanlog supports (or should support)

change the genny command to generate better fake logs, using more various realistic formats

• formats like found in https://github.com/humanlogio/humanlog/tree/master/test/cases
• and other loggers such as https://pkg.go.dev/go.opentelemetry.io/otel/log etc
• ideally not just Go logs, but we can start with that for now
• make sure its extensible so we can easily add more types of logs
• make sure the command generates a variety of formats with minimal fuss (dont want to have to use 1 million flags)
• at the same time, it should be possible to also specify exactly what type of logs we want to generate