Add field group_name or something similar

**Please describe what you are requesting**
There is no field for adding the name of user or system groups.

**Describe what change you are proposing**
Add a field like `group_name` for the group membership of user or system accounts.

**Describe the log source**
systemd
https://docs.nxlog.co/refman/current/im/systemd.html

**Attach any sample logs or examples for details**
There are many use cases where group names are provided as a context to the user which performed an action.

Review the systemd log source and the linked NXLog systemd documentation first, then locate where illuminate field definitions for user or system account context are maintained. Done means the relevant field is documented and supports group membership names from systemd logs.

Written by the indexing model from the issue text.