librarian: port exit gate package manifest creation code to Librarian

This should be able to be done in a language-neutral way.

See links in go/sdk:python-generation to current implementation in shell scripts.

Start at go/sdk:python-generation and follow the links to the current shell-script implementation. Compare how package manifest creation works there with Librarian’s existing CLI flow, then confirm that Librarian performs the same creation in a language-neutral way. Done means the exit-gate manifest creation is handled by Librarian rather than the shell scripts.

Written by the indexing model from the issue text.