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

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.

Start with CONTRIBUTING.md and compare the proposed sample-amazon-aurora-dsql-auth-session-mgmt/ layout with sample-amazon-aurora-dsql-agent/ and the existing typescript/ samples. Review PR #1216 alongside the listed authentication, session, migration, retry, and bulk-revocation patterns; done means the full HTTP sample app demonstrates these DSQL-specific patterns end to end.

Written by the indexing model from the issue text.