[layers v2] Spike — full micro-templates: merge a complete azd template into an existing project

Part of #9765.

Add in `azd add -t -p --layer ` command, which lets you merge an entire azd template's `azure.yaml` into a project's existing `azure.yaml`. This is the core workflow for a true micro-template — an entire azd project, imported into an existing one.

- Do we need a new file format, or is it enough to layer in merge guidelines on top of the existing schema? Different `azure.yaml` keys need different merge treatment (additive/namespaced, project-level singleton, "merge with care", or flatly incompatible).
- We want to avoid getting to "npm" levels of complexity (@vhvb1989 brought this up — what happens if you have templates that depend on templates, etc.). It can spiral, fast, so we need to set some guidelines here.
- We want to support importing a template by reference, not just vendoring the whole thing. This opens up a mode where a customer can just receive updates for a template instead of managing it themselves.
- Known complications:
- `services[].config` is an opaque, extension-owned blob (`additionalProperties: true`) — it either comes over wholesale, or we don't bring it over.
- Also needs to relocate `metadata.template` from the project level down to the layer level, since a project will eventually be assembled from several templates that each move independently, not just one.