[layers v2] Surface layer events in extension event streams

Part of #9765.

(fuzzy on this one, but I believe we deliver some event streams to extensions or consumers, so we need to just add in layer events as well, and/or alter relevant events to also carry layer information)

Figure out how layer-level events (added, provisioned, deployed, removed, etc.) show up in azd's existing extension event streams.

- Extensions need visibility into layer lifecycle events the same way they can observe other project events today.
- Should be designed alongside the layer-aware RPC API work so the event shapes stay consistent with how layers are represented elsewhere in the API.

Start by tracing azd’s existing extension event streams and reviewing the layer-aware RPC API work in #9765. Determine how current project events are exposed, then define layer lifecycle event visibility and shapes that match the API’s layer representation. Done means extensions can observe added, provisioned, deployed, and removed layer events consistently.

Written by the indexing model from the issue text.