Check for group existence before trying assignments

Ideally the script should check or ask the user for creation of groups to be used as assignment of configuration policies.

Currently it tries to find the group that is part of the JSON file, and then the assignment call fails with a bad request if the group doesn't exist.

The documentation in [Enterprise Access Model](https://learn.microsoft.com/en-us/security/privileged-access-workstations/privileged-access-deployment) is also mis-aligned with this Repo, for example, it talks about creating a group called "**Secure** Workstation Users" but the repo references "(**Specialized or Privileged**)" Workstation Users