[EPIC] Layered provisioning

This epic tracks our work to make azd's azure.yaml more modular — allowing composability (`azd add`) that works with layered provisioning, giving extensions a better API to work with layers, and extending the CLI to be layer-aware.

The various file formats (arbitrary, but it'll make it easier to discuss what we're changing to and from):

- **flat layout**: The existing format (pre-layers), where `infra:` and `services:` are root-level elements, is the "flat" layout.
- **layered provisioning v1**: The current layered provisioning format:

```yaml
services:

infra:
layers:
- name: layer-1
provider: bicep
```

- **layered provisioning v1.5**: The new layered provisioning format:

```yaml
services:

layers:
- name: layer-1
services:
serviceA:
host:
infra:
- name: infra-1
provider: bicep
```

This is a semi-long-term, multi-phase project under the "support extensions" initiative. See the linked child issues for the individual workstreams; expect this list to evolve as work progresses.