[FEATURE REQUEST] Ability to inject custom tags

### Ask

It would be amazing to have a way to set an environment variable in your dev environment with a list of custom tags that you want applied to any AZD deployment. It would be even better if there's a way to do this without modifying existing templates.

### Context

It's not uncommon to have requirements on your Azure subscription to tag your deployments with metadata like:

```
created-by:
deployment-type:
:
```

Today, I need to modify any AZD template I pull down to add this functionality.

In a perfect world, I could have either an environment variable or some special *.env file (bonus points for recursive lookup) where I can define custom tags that should be applied to any AZD deployment I create.

### Research

I searched the documentation and this repository to see if someone has brought this up before. If I missed another issue, I apologize.