(IAC-91) Postgres flexible server support

[manoatsas]

[In DRAFT] added changes to support Azure Postgresql Flexible Server for #79

This PR replaces https://github.com/sassoftware/viya4-iac-azure/pull/171 given the extent of changes to Postgresql since PR 171 was opened.

Changes here follows this https://docs.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-manage-firewall-portal uses 'Public access (allowed IP addresses)' option to configure Postgresql firewall rules for external access and to allow connections from Viya4 pods on Azure AKS and enables https://docs.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-manage-firewall-portal#connecting-from-azure. This follows the currently supported pattern for Azure Postgres Single server.