Add module for Azure PG flex server

[Ferdinanddb]

Add a new module to create an Azure Postgres Flexible Server and modify the AKS and Managed Identities modules. I listed the following in the issue associated to this PR.

Modification of the managed identities module

• Add a azurerm_user_assigned_identity that will be used by the e2e tests

Modification of the AKS module

• Add a azurerm_federated_identity_credential to map the newly created user assigned identity to the keda-operator Kubernetes service account.

New module for Azure Postgres Flexible Server

• Add a azurerm_postgresql_flexible_server,
• Add a azurerm_postgresql_flexible_server_active_directory_administrator to grant admin role to the newly created user assigned identity,
• Add a azurerm_postgresql_flexible_server_firewall_rule to allow access to all Azure IPs.
  • I am implementing it that way because the AKS cluster has an Azure IP, so I think it is the way to go, or maybe the range could be more specific? What do you think?
• Add a azurerm_postgresql_flexible_server_database
  • This database will be used by the e2e tests.

What do you think about this?

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EDIT: I decided to add support for basic Postgres authentication, so that the current way of performing actions in the server (creating a table, write to the table, ...) for the e2e tests can persist. But the ScaledObject of the e2e tests will authenticate as the user managed identity added by this PR to interact with the server.

There could be another way to handle this, but it is not as simple:

• Add a azurerm_postgresql_flexible_server_active_directory_administrator for the service principal used by Terraform,
• Add a azurerm_postgresql_flexible_server_firewall_rule to add the public IP (or range) of the machines from where the GitHub Actions workflows are executed. This is not necessary if Terraform is being executed from a resource having an Azure IP.
• Generate SQL queries and run them via null_resource resources + provisioner local-exec to add Azure Managed Identities, see this link for an example of the queries to run. This query must be executed by a user AD admin within Postgres (this is why the resource from the 1st bullet point is needed).
• With that, it would mean that basic authentication is disabled, and the only way to connect to the server is by using access token, meaning that the following code should be adapted to support authentication using Access Token. This is the code used by the container performing the e2e tests to create a table and perform other actions.

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)

Related to #151

[JorTurFer]

Looking really good, some comments inline

[Ferdinanddb]

@JorTurFer I took your comments in account and adapted the code accordingly :).

[zroubalik]

LGTM, great job

[Ferdinanddb]

@zroubalik Thanks for your feedback and merging it!

However, I see that the CICD stage that runs the Terraform apply command failed because of something I did not expect, it seems to be related to the Azure region being used or the quota of the subscription.

In my case, I used the Switzerland North region to test this locally and I only created an AKS cluster and the resources from this module.

Let me know if I can do something regarding this.