Allow for service-level provisions

[savannahostrowski]

For example, suppose I'm a new dev to the contoso real estate app. How do I provision only the cms service?

Originally posted by @diberry in https://github.com/Azure/azure-dev/discussions/2649

[savannahostrowski]

We have a similar pattern for deploy but not for provision.

[vhvb1989]

I talked about this to @ellismg some time ago. One approach here is by supporting azd-nested projects. From a root azure.yml, users could add a new service with type azd.
When calling azd provision, azd would use the top-level infra folder, and then it would call azd provision to each nested-azd project.

That would allow folks to split a big system with many apps.

[savannahostrowski]

I think that the issue we are going to have here is that this doesn't map 1:1 with how application developers think about their application. I think ideally we'd end up with deploy and provision behaving the same way because that's the easiest thing to understand but I recognize that's not really possible today.

Might be worth revisiting this after we think through IaC abstraction in Germanium.

[jongio]

We could do with one bicep, but hacky. It would be a mix of env var setting and checking. If --service=cms, then set env var, and then read that env var into the bicep and have a conditional in the bicep.