plugins/clnrest: Updated clnrest's requirements.txt

Even after migrating from pip install to poetry install for clnrest,
we still need to keep `plugins/clnrest/requirements.txt` for Fedora-28-amd64
build. `Dockerfile.builder.fedora` does not have poetry and will need
requirements.txt to install libraries with pip.

It is also required for non-developers who want to build cln from a tagged or master version.