[lib/ native] Rename fetchJSON to callServerEndpoint

Summary: Renames fetchJSON to callServerEndpoint and changes accompanying types to match.

Test Plan:

1. Run flow
2. Test some basic things in the native and web app to check if it works

Reviewers: tomek, atul, abosh

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5247