[Tunnelbroker] Add aws-sdk support, and cli configuration

Summary:
Add ability to target localstack endpoint, and
add initial support for aws-sdk.

Part of:

• https://linear.app/comm/issue/ENG-3822
• https://linear.app/comm/issue/ENG-3671

Test Plan:

cd services/tunnelbroker
# --help should emit usage now
cargo run -- --help

Reviewers: bartek, varun

Reviewed By: bartek, varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7715