[feature-flags] Standardize localstack and http config

Summary:

• Replaced old --sandbox flag with --localstack-endpoint that we use in other services. More info in D8835
• Also renamed --port to --http-port to match other services
• Changed default port to 50055, see https://linear.app/comm/issue/ENG-3288/dont-bind-services-to-default-grpc-port

Test Plan:
cargo run - should start with default localstack endpoint (LOCALSTACK_ENDPOINT env is set by Nix). Default HTTP port is 50055.
The --http-port and --localstack-endpoint arguments do override the defaults.

Reviewers: michal, tomek, varun, jon

Reviewed By: michal

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9138