[Feature]: Request to add a ResourceGroupName and ResourceGroupId property to PowerShell Command Get-AzPostgreSQLFlexibleServer

### Description of the new feature

ResourceGroupId and/or ResourceGroupName are fundamental attributes when working with Azure.
In the case of powershell for PostgreSqlFlexibleServer, both New-PostgreSqlFlexibleServer & Get-PostgreSqlFlexibleServer do not include the propertys ResourceGroupName nor ResourceGroupId, but those are mandatory for example for Set-PostgreSqlFlexibleServer.
As reference, the cmlt Get-PostgreSqlServer includes the property ResourceGroupName.

Example of what this change would allow:
$newserver = Get-New-PostgreSqlFlexibleServer ...
$newserver | Set-PostgreSqlFlexibleServer ...

### Proposed implementation details (optional)

Add output propertys ResourceGroupName and ResourceGroupId to Get-PostgreSqlFlexibleServer and New-PostgreSqlFlexibleServer

Start by locating the implementations and output models for Get-PostgreSqlFlexibleServer and New-PostgreSqlFlexibleServer. Compare their properties with Get-PostgreSqlServer, then verify that ResourceGroupName and ResourceGroupId are available for piping into Set-PostgreSqlFlexibleServer.

Written by the indexing model from the issue text.