[Nix] Add localstack-down

Summary:
Allow for localstack to be easily be shutdown through nix

part of: https://linear.app/comm/issue/ENG-1697

Depends on D5245

Test Plan:

nix run .#localstack-up
# get message about it being up
docker ps # assert localstack is running

nix run .#localstack-down
docker ps # assert localstack is no longer running

Reviewers: atul, abosh, varun, O1 Nix, ashoat

Reviewed By: varun, O1 Nix, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5246