release-782ef18: Initial Loom python utility script

Summary:
Adds
 - struct for `DeviceTrace`
 - function to list traces on device
 - ability to pull a specific trace

Notes
 - error handling is minimal, happy to add something more robust but preferred to put up something more minimal to start.
 - the package name needs to be provided. I could grab the package of the focused activity if we want to add that. It felt more hacky than I'd like and it's not required so I omitted it for now.

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Reviewed By: BurntBrunch

Differential Revision: D6425982

fbshipit-source-id: 16e4f93dbe68a32c6a13217cc3831764f348e0bf