Hammer CLI

Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1680053

Description of problem:
Currently it's not possible to also show the gateway address with 'hammer subnet list' - example output: # hammer subnet list
---|----------|--------------|---------------|--------
ID | NAME | NETWORK | MASK | VLAN ID
---|----------|--------------|---------------|--------
1 | net-int1 | 192.168.40.0 | 255.255.255.0 |
4 | net-dmz2 | 192.168.51.0 | 255.255.255.0 |
2 | net-int2 | 192.168.41.0 | 255.255.255.0 |
---|----------|--------------|---------------|--------

As reference, the gateway address is available with 'hammer subnet info': # hammer subnet info --id 1 | grep Gateway
Gateway: 192.168.40.1

Version-Release number of selected component (if applicable):
Satellite 6

Actual results:
It's not possible to show the gateway address with 'hammer subnet list'.

Expected results:
Also show the gateway address with 'hammer subnet list' by default, or as an option.

Additional info:
The following upstream RFE might be relevant:

Possibility to limit or add fields that are displayed
https://projects.theforeman.org/issues/19135