rpk: compose --cluster-type with --dashboard instead of conflicting

Rework --cluster-type stretch from an embedded-only override that
rejected --dashboard into an overlay of the default dashboard set: the
stretch variant replaces the operations dashboard, while any dashboard
without a stretch-specific variant is generated as usual. Stretch
variants now follow the same flow as the other dashboards — downloaded
from the observability GitHub repository, with the embedded gzipped
copy as fallback pinned by sha256.

The download host moved to a package variable so unit tests can stub
it; tests now cover the GitHub path, the embedded fallback, and the
overlay fallback to the default set.

Ref K8S-855

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>