Allow specifying annotations on ArgoCD projects

[Dragemil]

https://argo-cd.readthedocs.io/en/stable/operator-manual/notifications/subscriptions/

Our slack sender should be configured?

[Copilot]

Pull Request Overview

This PR adds support for custom annotations on ArgoCD applications to enable notification configurations. The change allows users to specify annotations like those needed for ArgoCD notifications to Slack as mentioned in the ArgoCD documentation.

• Adds a new annotations variable to accept custom annotations for ArgoCD applications
• Updates the ArgoCD application resource to use the provided annotations
• Documents the new variable in the module readme

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
argocd_app/variables.tf	Adds new annotations variable with map(string) type and empty default
argocd_app/app.tf	Applies the annotations variable to the ArgoCD application metadata
argocd_app/readme.md	Documents the new annotations input parameter

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[marcin-dardzinski]

Since we're configuring it for our needs, maybe it's better to have a more specific optional slack_notifications_channel param.

Generic labels/annotation could be kept as well for any extravagant needs