feat: add TypeScript SDK alongside Python SDK

[saksham-nexla]

Summary

This PR introduces a new TypeScript SDK for Nexla, providing feature parity with the Python SDK for core operations. The TS SDK enables JavaScript/TypeScript developers to integrate Nexla into their applications seamlessly.

Key Features

• NexlaClient with service key and access token authentication
• Generated OpenAPI client from official Nexla OpenAPI spec
• Type-safe resource clients for common operations (flows, sources, destinations, etc.)
• Webhooks support for webhook signature verification and one-record sends
• Comprehensive test coverage (unit tests with 90%+ coverage)
• Automated CI/CD for testing, linting, and publishing

Test Coverage

• Unit tests for all resource clients
• Integration test infrastructure (non-blocking CI)
• Coverage reporting enforced via CI checks

Documentation

• Architecture overview
• API parity matrix (Python ↔ TypeScript)
• Migration guide from Python SDK
• Integration testing policy

Monorepo Structure

• packages/ts-sdk/ - TypeScript SDK source code
• docs/ts-sdk/ - TypeScript-specific documentation
• scripts/parity/ - Python/TS parity tooling

Checklist

• All tests pass (unit + lint)
• Documentation updated
• Type checking enforced (no any types)
• CI workflows configured

[saksham-nexla]

Renaming branch - will open new PR with corrected branch name