Provide set of avaiable features for UserGroup

[JimOverholt]

Description

Set of features which can be administered per user group is dynamic. Provide dedicated endpoint which will return all possible features for given vmanage instance. Features are avaiable via /admin/usergroup under headers key in JSON response.

{
    "header": {
        ...
        "viewControl": [
             ...
            {
                "keyProperty": "tasks",
                "dataType": "collection",
                "controlType": "ReadWritePermissionView",
                "label": "Tasks",
                "controlData": {
                    "type": "local",
                    "data": [
                        {
                            "feature": "Alarms",
                            "displayName": "Alarms"
                        },
                        {
                            "feature": "Application Monitoring",
                            "displayName": "Application Monitoring"
                        },
                        {
                            "feature": "Audit Log",
                            "displayName": "Audit Log"
                        },
                        ...
                       {
                            "feature": "Policy Configuration",
                            "children": [
                                {
                                    "feature": "ACL Policy",
                                    "displayName": "Policy - ACL"
                                },
                        ...
                        {
                            "feature": "Feature Profile > SSE > Zscaler SSE",
                            "displayName": "Feature Profile > SSE > Zscaler SSE"
                        },
...

Model needs to provide helper method which will return flat Set[str] compatible with with how the features are modified in user group using POST/PUT.

[JimOverholt]

This issue was migrated from an old repository: https://github.com/CiscoDevNet/catalystwan/issues/443