feat(oauth): implement client credentials flow and authenticated webhook subscriptions (#92)

[annukumar123]

🔑 Overview

Implements standard OAuth client credentials authentication mechanics inside the core client factory to support authenticated lifecycle tracking across Webhook & Event Pub/Sub components.

🛠️ Changes Introduced

• Added client.oauth.authenticate(clientId, clientSecret) to exchange application credentials for runtime access tokens.
• Added client.oauth.subscribeWebhook(targetUrl, events) for managing secure event delivery paths.
• Updated internal request transport context to dynamically inject OAuth bearer tokens over static api keys when running token-validated operations.
• Added corresponding suite variations inside test/sdk.test.mjs confirming full end-to-end functionality.

#92