Endpoint to get existing and prospective resources for service connector (#2854)

* endpoint to get existing and prospective resources for service connector

* simplify if-else

* Update src/zenml/service_connectors/service_connector_utils.py

Co-authored-by: Michael Schuster <[email protected]>

* Update src/zenml/zen_server/routers/service_connectors_endpoints.py

Co-authored-by: Michael Schuster <[email protected]>

* review suggestions

* GE mypy

* review comments -> dict object

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Co-authored-by: Michael Schuster <[email protected]>

Author: avishniakov