[FEAT] - improve help message for mgc network security-groups rules create option

[ximenesyuri]

Is your feature request related to a problem? Please describe.

1. the flag --direction seems to be a mandatory field. However, it is not displayed as "required"
2. the flags --direction, --ethertype and --protocol assumes certain values, which are not described in the help message

Describe the solution you'd like

1. to mark --direction as a required field
2. to explicit the possible values for the listed fields: --direction (ingress or egress), --ethertype (IPv4 or IPv6) and --protocol (icmp, icmpv6, tcp or udp)

Additional context
Currently the user discover:

1. that --direction is mandatory
2. the possible values of the listed fields

only through error messages.