Standardize ElasticsearchRetriever (#94)

* remove es_params, init acccepts clients and creds

* update tests

* add docs and examples

* update tests

* Remove test_retriever_examples.py from tracking

* Remove test_retriever_examples.py (sync) from tracking

* lint

* fix trailing whitespace

* fix mypy types error

* fix more mypy type errors

* remove comment

Author: ssh-esh