Add spec for Networking (#24892)

Summary:
Part of #24875, adds a spec for Networking. Since `sendRequest` methods are different for both platforms, I had to create 2 spec files as Flow would merge their definitions even when I added `Platform.OS` check

## Changelog

[General] [Added] - TM spec for Networking
Pull Request resolved: https://github.com/facebook/react-native/pull/24892

Reviewed By: RSNara

Differential Revision: D15543067

Pulled By: fkgozali

fbshipit-source-id: 2b91114dfa45e7899bbb139656a30a6fd52e31db