feat(parser): add comprehensive Cognito trigger schemas

- Implemented 12 new Zod schemas for Cognito Lambda triggers:
  - Pre-Signup, Post-Confirmation, Pre-Authentication, Post-Authentication
  - Pre-Token Generation (V1 & V2), Migrate User
  - Custom Message, Custom Email Sender, Custom SMS Sender
  - Define Auth Challenge, Create Auth Challenge, Verify Auth Challenge
- Updated schemas/index.ts for dedicated exports
- Added unit tests in cognito.test.ts following Prepare/Act/Assess format

Author: VatsalGoel3