Add OAuth-based authentication to ServiceNow integration with basic auth fallback

[qodo-code-review]

Description:

• The ServiceNow integration must support OAuth authentication to align with modern security practices and ServiceNow recommendations.
• Users should be able to connect using OAuth tokens without storing long-lived passwords, improving security posture.
• The integration must continue to support existing basic authentication to avoid breaking current setups.
• Configuration should adapt based on availability of OAuth data, making username/password optional when OAuth is configured.
• The client should automatically use OAuth Bearer tokens when present, refresh credentials per request, and fall back to basic auth if needed.

Deliverables:

• OAuth support: Enable OAuth authentication flow for the ServiceNow integration, using access tokens for API requests.
• Backward compatibility: Maintain basic auth support and make it the fallback when OAuth is not enabled or tokens are unavailable.
• Config updates: Update integration config so servicenowUsername and servicenowPassword are optional when OAuth is configured, and map OAuth profile data to these fields in saas settings.
• Auto headers: Ensure request headers automatically include the correct Authorization scheme (Bearer or Basic) and refresh per request when needed.
• URL override: Populate servicenowUrl from the OAuth instance URL when available, with a safe fallback to the configured value.