Feature request: Support Express mode

[khmoryz]

AWS released CloudFormation Express mode last month.
It makes deployments faster without requiring any template changes.
This aligns well with rain's philosophy.

It's already supported by aws-sdk-go-v2 2026-06-30 release (need sdk bump).
I think the changes will mainly be passing DeploymentConfig.

Would you be open to this feature?
I'd be happy to submit a PR.

Thanks for taking a look :)

Ref:

• Accelerate your infrastructure deployments by up to 4x with AWS CloudFormation Express _mode
• Deploy AWS CloudFormation stacks faster with express mode