Add Terraform module for PostgreSQL Flexible Server

[jarkkoka]

• [x] Copy Terraform module definitions from https://dev.azure.com/hslfi/Azure-automation/_git/Terraform-modules
• [x] Find a way create admin credentials for the PostgreSQL server into Azure key vault
• [ ] Customise Terraform module according to Jore4 requirements (e.g. subnet, private DNS zone)
• [ ] Test SSH connectivity from developer's computer to Azure PostgreSQL database using Azure Bastion service (for importing database dump)