CLI: Missing set action for custom attributes

lqb

Description of the problem, including code/CLI snippet

There isn't an action available to set custom attributes in the CLI.
The commands group-custom-attribute, project-custom-attribute and user-custom-attribute are affected.

Expected Behavior

The commands should have actions to set custom attributes

$ gitlab group-custom-attribute -h
usage: gitlab group-custom-attribute [-h] {list,get,set,delete} ...

options:
  -h, --help         show this help message and exit

action:
  {list,get,delete}  Action to execute on the GitLab resource.
    list             List the GitLab resources
    get              Get a GitLab resource
    set              Set a Gitlab resource
    delete           Delete a GitLab resource

Actual Behavior

The actions to set custom attributes are missing

$ gitlab group-custom-attribute -h
usage: gitlab group-custom-attribute [-h] {list,get,delete} ...

options:
  -h, --help         show this help message and exit

action:
  {list,get,delete}  Action to execute on the GitLab resource.
    list             List the GitLab resources
    get              Get a GitLab resource
    delete           Delete a GitLab resource

Specifications

python-gitlab version: 8.3.0
Gitlab server version (or gitlab.com): 8.11.2