Deprecate PG support and route to new PG extension

[sevoku]

Since there is a new PostgreSQL extension with full support for Azure, we're not planning to keep PG support with Azure Databases.

Deprecation steps:

• validate that "Azure Cosmos DB for PostgreSQL" accounts are fully supported by the new extension!
• discuss with the PG team if they want to add support for Azure Resources to replace our existing nodes
• if yes, work with the Azure Tools and PG teams to replace the registered nodes in Azure Resources
• if not, evaluate if we can route to the new extension when our PG nodes get activated
  • The PG extension has a URL handler and a "Connect with VS Code" feature in the Portal:
    
    Maybe we can generate a vscode:// url and let VS Code handle it to add a PG connection.
  • Ask the PG team to add a command which could be called by our extension to add a connection
• register a new "Azure Cosmos DB for PostgreSQL" resource type in Azure Resources with the same behaviour (see Azure Cosmos DB for PostgreSQL Cluster not showing in Azure Resources list #2592)
• add a new "Connect with VS Code" feature for "Azure Cosmos DB for PostgreSQL" accounts in Portal (cc @jawelton74)