[`azure-appgw`] Add configurable block `managed_rules.exclusions`

[JulianGV21]

Motivation

Following the structure described in:

https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/web_application_firewall_policy#exclusion-1

we need to update the configuration in:

https://github.com/prefapp/tfm/blob/main/modules/azure-app-gateway/web_application_firewall_policy.tf

Currently the module supports:

• policy_settings
• custom_rules
• managed_rule_set
• rule_group_override

but it does not support managed_rules.exclusion.

We need to create a dynamic block that supports multiple exclusions inside managed_rules.

Acceptance criteria

• Add support for managed_rules.exclusion
• Support multiple exclusions through dynamic blocks
• Keep backward compatibility with existing WAF configurations
• Update variable schema accordingly
• Add example usage in module documentation

[gustavoborragan]

This PR addresses and resolves the issue #1284.
Ready for review!

[gustavoborragan]

#1193
https://github.com/prefapp/tfm/releases/tag/azure-app-gateway-v0.9.0