Enable azd Composability

[savannahostrowski]

We need to invest in supporting developers beyond their initial onboarding to Azure, as they are iterating on their project. This includes both application source code and infrastructure.

We should add capabilities to allow the CLI to support developers as they make updates to existing infrastructure in a scaffolded project (e.g., the ability to swap out a database, update a configuration etc.).

Not only is this valuable for those who understand the ins and outs of Azure offerings enough to want to switch or update a resource but it also lessens the cost to developers (in terms of time etc.) to try out a template or piece of technology. A la carte infrastructure means that a developer has true flexibility beyond day 0 and reserves the right to change their mind or update infrastructure they are using for their project if their requirements change etc.

[savannahostrowski]

Some demand for similar via https://github.com/Azure/azure-dev/discussions/1218

[aaronpowell]

Additional use-case Feature request - add template to existing project