flattening structures for use by other AsyncApi spec tools

[bluemonk3y]

See #221 - example command to flatten:

% docker run --rm -v "$(pwd)/resources:/app" ghcr.io/specmesh/specmesh-build-cli flatten --in-spec /app/simple_schema_demo-api.yaml --out-spec /app/testing.yaml

Changes:

channels:
  # PRODUCER/OWNER build pipe will publish schema to SR
  _public.user_signed_up:

into

channels:
  simple.schema_demo._public.user_signed_up:

[bluemonk3y]

lgtm