feat: add s3-bucket and s3-prefix inputs for large template upload

[kddejong]

Summary

Adds s3-bucket and s3-prefix inputs so that large templates exceeding the 51,200 byte inline limit can be automatically uploaded to S3 before creating the change set.

When s3-bucket is provided and the template is a local file, the action uploads it via PutObject and passes TemplateURL instead of TemplateBody to CloudFormation.

Closes #180

Changes

• action.yml: Added s3-bucket and s3-prefix inputs
• package.json: Added @aws-sdk/client-s3 dependency
• src/main.ts: S3 upload logic when s3-bucket is provided
• src/validation.ts: Added new inputs to validation schemas
• __tests__/main.test.ts: 3 new tests covering S3 upload with/without prefix and URL passthrough

Usage

- name: Deploy large template
  uses: aws-actions/aws-cloudformation-github-deploy@v2
  with:
    name: MyStack
    template: my-large-template.yaml
    s3-bucket: my-deployment-bucket
    s3-prefix: cfn-templates

Testing

All 209 tests pass.