[blob-service] Standardize localstack config

Summary:
We should have one standard way of configuring localstack endpoint for all services. This diff standardizes the localstack config for blob-service.
I consider Tunnelbroker as state-of-the-art solution for localstack endpoint configuration.

TL;DR

cargo run -- --sandbox --localstack-url "http://localhost:4566"

-->

cargo run

The LOCALSTACK_ENDPOINT env variable is automatically provided by Nix so no need to specify --localstack-endpoint explicitly.

Test Plan: Ran the above command and it worked. Blob service integration tests are working.

Reviewers: michal, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8835