feat: Add CloudFormation Language Extensions support (Fn::ForEach)

[bnusunny]

Description

This PR adds support for CloudFormation Language Extensions in SAM CLI, addressing GitHub issue #5647.

Features

• Fn::ForEach - Iterate over collections to generate resources
• Fn::Length - Get the length of an array
• Fn::ToJsonString - Convert objects to JSON strings
• Fn::FindInMap with DefaultValue - Map lookups with fallback values
• Conditional DeletionPolicy/UpdateReplacePolicy - Use intrinsic functions like Fn::If in resource policies

Key Design Decisions

1. In-Memory Expansion Only - Templates are expanded in memory for SAM CLI operations, but the original unexpanded template is preserved for CloudFormation deployment
2. Dynamic Artifact Properties via Mappings - Fn::ForEach blocks with dynamic artifact properties (e.g., CodeUri: ./src/${Name}) are supported via a Mappings transformation
3. Locally Resolvable Collections Only - Fn::ForEach collections must be resolvable locally; cloud-dependent values (Fn::GetAtt, Fn::ImportValue) are not supported with clear error messages

Supported Commands

• sam build - Builds all expanded functions, preserves original template
• sam package - Preserves Fn::ForEach structure with S3 URIs
• sam deploy - Uploads original template for CloudFormation to process
• sam validate - Validates language extension syntax
• sam local invoke - Invokes expanded functions by name
• sam local start-api - Serves ForEach-generated API endpoints
• sam local start-lambda - Serves all expanded functions

Example

Transform:
  - AWS::LanguageExtensions
  - AWS::Serverless-2016-10-31

Resources:
  Fn::ForEach::Functions:
    - Name
    - [Alpha, Beta, Gamma]
    - ${Name}Function:
        Type: AWS::Serverless::Function
        Properties:
          Handler: ${Name}.handler
          CodeUri: ./src
          Runtime: python3.9

Resolves #5647

Testing

• Comprehensive unit tests for the language extensions engine
• Property-based tests using Hypothesis
• Integration tests for all supported commands
• Test templates covering static/dynamic CodeUri, nested stacks, parameter collections

Checklist

• Unit tests added
• Integration tests added
• Documentation in code comments
• Error messages include actionable workarounds

[bnusunny]

Integration test: https://github.com/aws/aws-sam-cli/actions/runs/21808626015