Add a FinalizeExport RPC to pass the combined exporter response back to
the session provider after all exports complete. This lets clients run
completion work while the build's gateway references remain available.
Keep the callback optional for compatibility with existing session
exporters. Ignore unsupported finalization, propagate callback failures,
and support providers that only register a finalization callback.
Signed-off-by: Alberto Garcia Hierro <alberto.hierro@docker.com>
30a3c27393