Add versioned Lambda asset uploads

[iloewensen]

Summary

Adds an opt-in Lambda code asset path controlled by SST_LAMBDA_CODE_ASSET_VERSIONING.

When enabled, SST uploads Lambda zip assets to a stable S3 key and updates the existing BucketObjectv2 using sourceHash. Lambda functions then reference the uploaded object version through s3ObjectVersion.

Motivation

Large deployments with many Lambda functions currently create a new S3 object key for every function code change. In testing with 250 functions, this caused significant S3 object churn and larger Pulumi state updates.

Measured results from the investigation:

• Current behavior, 250 functions: ~4m22s
• Versioned Lambda assets, 250 functions: ~2m43s
• No code changes: ~13s

CloudTrail and Pulumi debug logs did not show AWS throttling as the primary bottleneck. The remaining cost appears to be Lambda control-plane update latency across many functions.

Changes

• Adds SST_LAMBDA_CODE_ASSET_VERSIONING=1 / true.
• Uses stable Lambda asset keys under lambda/<app>/<stage>/<function>/code.zip.
• Sets sourceHash on the BucketObjectv2 so Pulumi updates the same object when code changes.
• Passes s3ObjectVersion to Lambda so code updates point at the exact uploaded object version.
• Enables versioning on the SST asset bucket during AWS bootstrap.
• Adds lifecycle rules to keep Lambda asset versions bounded.

Compatibility

Default behavior is unchanged when SST_LAMBDA_CODE_ASSET_VERSIONING is not set.

Dev bridge code keeps the existing hashed assets/dev-bridge-code-<hash>.zip behavior.