Add Retriever to llama-index-networks (#11800)

* refactor

* update demo

* update simple

* version bump prerelease

* update demo nb

* NetworkRetriever class and contributor files

* linting fix

* bump pre-release version

* bump pyproject.toml to release

* client and service rewrite

* add unit tests

* fixed init

* get tests to run -- still fail

* fix api

* revert back to mocking requests.post'

* passing test_retriever_service

* fix network retriever to use query bundle; passing unit test

* build files

* build files

* rename and add backwards compat for v0.1.0 imports

* update simple demos

* update demo

* move aliases in the module and import them into __init__

* update node registry

---------

Co-authored-by: Izuki Matsuba <imatsuba@omers.com>