[services] Update AWS Rust SDK

Summary:
Updated AWS SDK for Rust. Most notable changes:

• All builders now return Result - they fail only on programmer error - when required items are not set, so used expect()
• Needed to upgrade siwe crate to resolve dependency conflict

Other changes described in inline comments

Depends on D12878

Test Plan:

• Cargo build for the whole workspace
• CI, including Commtest
• Manual testing locally for Identity and Blob

Reviewers: varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12879