bin/enqueue-job: Use clap to parse commands and generate help text

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Usage: enqueue-job <COMMAND>

Commands:
  update_downloads      
  dump_db               
  daily_db_maintenance  
  squash_index          
  normalize_index       
  help                  Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help information

Usage: enqueue-job dump_db <DATABASE_URL> [TARGET_NAME]

Arguments:
  <DATABASE_URL>  [env: READ_ONLY_REPLICA_URL]
  [TARGET_NAME]   [default: db-dump.tar.gz]

Options:
  -h, --help  Print help information