Add S3 Express cache layer examples with spawn workers

[ksmotiv8]

Summary

Adds three new examples demonstrating a write-through cache pattern with async durability using S3 Express One Zone:

• s3-express-upload.rs: Upload function that writes to Momento Cache synchronously and spawns an async worker for S3 durability
• s3-express-worker.rs: Spawn function that handles async S3 uploads with full AWS SigV4 signing and metrics logging to topics
• s3-express-download.rs: Cache-first download with S3 fallback

Key features demonstrated

• AWS SigV4 signing from scratch for S3 Express (service name: s3express, not s3)
• spawn!() vs post!() macros for spawn vs web functions
• Logging metrics to Momento Topics for observability of async workers
• Runtime environment variables via -E flag at deploy time
• Server-side timing instrumentation

Architecture

Client Request
      |
Upload Function
   |         |
Cache     spawn()
(sync)       |
         Worker
            |
       S3 Express
        (async)

No credentials included

All AWS credentials are read from environment variables at runtime using std::env::var().

Test plan

• Verify examples compile: cargo build --examples
• Review code for any hardcoded credentials
• Test deployment with actual AWS credentials via -E flags

Generated with Claude Code