[lib] Introduce invertObjectToMap

Summary:
Context: https://linear.app/comm/issue/ENG-5640/consider-introducing-invertobjecttomap-utility-fn

Thanks @tomek for investigating + resolving the flow issues with this utility!

Test Plan: Consumed in minimally-encoded-thread-permissions, where the unit tests continue to succeed.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Subscribers: wyilio, tomek

Differential Revision: https://phab.comm.dev/D9676