feat(serdes): Add filesystem serdes for large payload storage

[ayushiahjolia]

Issue #, if available: #479

Description of changes:

• Adds create_filesystem_serdes() - a SerDes implementation that stores serialized values on a durable filesystem (Amazon S3 Files or Amazon EFS mounted to Lambda). This keeps checkpoint sizes small regardless of payload
  size, with optional inline preview for observability.

Features -

• ALWAYS mode: Every value written to file; checkpoint stores file pointer
• OVERFLOW mode: Inline if < 256KB, overflow to file if larger
• URI path encoding: Human-readable paths derived from ARN (<functionName>/<executionName>/<invocationId>/<operationId>.json)
• HASH path encoding: Fixed-length SHA-256 paths for safety
• Preview: Optional inline preview in checkpoint envelope for console/API visibility without reading the full file
• ARN parsing: Compact directory structure from durable execution ARNs

Tests -

• Added unit tests, e2e tests with replay and integration tests.

Next PRs -

• Cloud integration tests (requires S3 Files or EFS mount infrastructure)
• Documentation update in aws-durable-execution-docs repo

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