Provide help messages for CLI

[phil-opp]

Example output:

Start the given dataflow path. Attach a name to the running dataflow by using --name

Usage: dora start [OPTIONS] <PATH>

Arguments:
  <PATH>  Path to the dataflow descriptor file

Options:
      --name <NAME>                          Assign a name to the dataflow
      --coordinator-addr <COORDINATOR_ADDR>  
      --attach                               Attach to the dataflow and wait for its completion
      --hot-reload                           Enable hot reloading (Python only)
  -h, --help                                 Print help

I didn't add help output for --coordinator-addr yet because I want to implements some other improvements there.

[haixuanTao]

Looks great! Thanks!