Flow: use cobra for the CLI and add an `agent run` command

[rfratto]

This changes

ENABLE_FLOW_EXPERIMENTAL=true agent --config.file=/path/to/file.river

to

ENABLE_FLOW_EXPERIMENTAL=true agent run /path/to/file.river

Help text is added to help users understand basic information about Flow.

Additionally, ENABLE_FLOW_EXPERIMENTAL=1 is also accepted to enable Flow.