[Nix] Add localstack-up

Summary:
Allow for localstack to be easily started through nix

Depends on D5244

Test Plan:

nix run .#localstack-up
# should get a message about localstack starting
docker ps # should see localstack in list

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

Reviewed By: O1 Nix, ashoat, atul

Subscribers: atul, ashoat, tomek

Differential Revision: https://phab.comm.dev/D5245