Proposal: add auth and session management sample (#1216)

[charycha]

Hi maintainers — opening this issue per CONTRIBUTING.md guidance for significant
contributions, alongside PR #1216.

Summary

Proposing a new top-level sample app: sample-amazon-aurora-dsql-auth-session-mgmt/,
following the convention used by sample-amazon-aurora-dsql-agent/.

Node.js + TypeScript service demonstrating user authentication and session
management on Amazon Aurora DSQL, with patterns specific to DSQL's distributed
architecture.

Patterns demonstrated

• IAM-based DB auth via @aws/aurora-dsql-node-postgres-connector
• Application-layer UUID generation, no foreign keys, no JSONB
• OCC retry with exponential backoff + jitter (SQLSTATE 40001)
• CREATE INDEX ASYNC, one-DDL-per-transaction migrations
• bcrypt password hashing, SHA-256 session token hashing
• 3,000-row transaction limit handling for bulk session revocation

Relation to existing samples

Companion to a Builder Center blog post (forthcoming) demonstrating these
patterns end-to-end. Different from the existing typescript/ ORM samples
in that it's a full sample app with HTTP routes, middleware, services, and
repositories — not a connector-only example.

PR: #1216

Happy to discuss the approach or folder placement before review.