Enable RPs & users to announce upcoming changes

I was unable to find any related issue, so here it goes:

I'd like to request a feature to enable users (and ideally also the Azure Resource Providers) to issue warnings about upcoming changes. The idea would be that you deploy a template (for example DiagnosticSettings as a very recent [example](https://learn.microsoft.com/en-us/azure/azure-monitor/essentials/migrate-to-azure-storage-lifecycle-policy)) and because of either something configured somewhere by the PG, or maybe the original creator of the template, a warning should show up in the logs during the deployment of the template.

This could look similar to how it is done in
- Terraform: [source](https://global.discourse-cdn.com/hashicorp/original/3X/b/4/b4ded7e6dd231dc99fed8628fa7fd720fbf24071.png)
- PowerShell: [source](https://stackoverflow.com/questions/57562831/how-do-i-get-rid-of-azure-powershell-warning-warning-azurevm-a-property-of-the)
> WARNING: Set-AzureRmVMBootDiagnostics: A property of the output of this cmdlet will change in an upcoming breaking change release. The StorageAccountType property for a DataDisk will return Standard_LRS and Premium_LRS
- Azure CLI: [source](https://miro.medium.com/v2/resize:fit:1400/1*OG6edhTM_R6L9OYptBohiA.png)

I could see this being quite useful also for, for example, the Public Bicep Registry, or any Solution Maintainer, to notify consumers of changes ahead of time - simply by showing them custom messages in the log.

I'm aware that I'm essentially asking for 2 things here:
- Enable template creators to embedd info messages in their templates that show up during deployments
- Enable RPs to issue warnings of upcoming changes that show up during deployments

Having the first would already be great. The second then the icing on the cake - especially in response to those aformentioned changes in the backend with the diagnostic settings which all of a sudden started breaking all over the place if you didn't happen to read about the change in the docs ahead of time.

The issue names no files, tests, or entry points. Start by separating template-authored messages from Azure Resource Provider warnings, then review how deployment logs currently surface diagnostics. Done means a decided scope and implementation path for showing upcoming-change warnings during deployments.

Written by the indexing model from the issue text.