[comm-lib] Hide aws behind a feature flag

Summary: While we used aws dependencies in all services, future diffs will make comm-lib a (transitive) dependency of native_rust_library. These dependencies aren't needed there so I'm introducing a new feature flag that hides them and the modules that depend on aws crates.

Test Plan: Check that all services compile

Reviewers: bartek, varun, kamil

Reviewed By: bartek, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10258