Use az cli instead of AZ Connect in powershell

[placerda]

https://github.com/Azure/GPT-RAG/blob/6b3b485159607bb20d60daee17dbc849f9306318/scripts/rai/raipolicies.ps1#L22

Update ps1 RAI script to use az cli to get token, the same way it is in the bash version, the idea is to have the same for both platforms.

token=$(az account get-access-token --tenant "$Tenant" --query accessToken --output tsv)