Implement Serverless Valkey cache with IAM authentication for MITxOnline

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This PR implements a complete Serverless Valkey cache solution for MITxOnline as a Redis replacement, addressing the transition requirements outlined in issue #3562.

Key Changes

New ServerlessCache Component

• Created OLAmazonServerlessCache component with full IAM authentication support
• Uses Valkey 8.x engine as specified in AWS Serverless ElastiCache requirements
• Implements proper validation for cache names, subnet limits (max 3), and configuration parameters
• Replaces traditional auth token authentication with IAM-based access

MITxOnline Integration

• Integrated serverless cache alongside existing Redis infrastructure (additive approach)
• Created ElastiCache IAM user (mitxonline-app-{env_suffix}) with full access permissions
• Established ElastiCache user group (mitxonline-usergroup-{env_suffix}) for proper authentication
• Updated IAM policies to include ElastiCache permissions: Connect, DescribeServerlessCaches, DescribeUsers, DescribeUserGroups

Authentication Architecture

The implementation addresses the IAM authentication requirements mentioned in the issue comments:

# Traditional Redis uses auth tokens
redis_cache_config = OLAmazonRedisConfig(
    auth_token=redis_config.require("password"),
    # ...
)

# Serverless cache uses IAM authentication
serverless_cache_config = OLAmazonServerlessCacheConfig(
    user_group_id=elasticache_user_group.user_group_id,
    # No auth_token needed
)

Exports and Integration Points

Added comprehensive exports for application configuration:

• serverless_cache_endpoint - Primary write endpoint for applications
• serverless_cache_reader_endpoint - Read replica endpoint for read operations
• serverless_cache_arn - Full ARN for resource references and policies
• serverless_cache_user_id / serverless_cache_user_group_id - IAM authentication credentials

Testing

• Comprehensive test suite for OLAmazonServerlessCacheConfig validation
• Tests cache name constraints, length validation, and configuration defaults
• Ensures proper error handling for invalid configurations

Technical Implementation Details

The serverless cache configuration handles AWS-specific constraints:

• Uses subnet_ids[:3] due to AWS 3-subnet maximum for serverless caches
• Implements proper Pulumi Output dependency handling for IAM policy generation
• Maintains backward compatibility with existing Redis infrastructure

This implementation provides the foundation for transitioning MITxOnline from traditional Redis to Serverless Valkey while maintaining the existing infrastructure during the migration period.

Fixes #3562.

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