Notion node gains OAuth2 authentication support

The Notion node previously only supported API Key authentication, which works for many use cases but lacks the token refresh capabilities and granular permissions that OAuth2 provides. This PR adds OAuth2 as a second authentication option, letting users choose the method that fits their security requirements.

Users who prefer OAuth2 can now connect their Notion integration using that flow, which is particularly valuable for long-running workflows where token expiration could otherwise interrupt operations. The implementation dynamically selects the credential type based on the chosen authentication method — API Key users continue using notionApi credentials while OAuth2 users get notionOAuth2Api credentials.

This change touches the core Notion node in the n8n nodes-base package, with modifications across the trigger node, the versioned operation node, and the shared authentication logic. The OAuth2 support was already partially prepared in the codebase — the options were commented out — and this PR activates that infrastructure.