Add platform-specific integration user recommendations

Summary

The Credentials page already recommends creating a dedicated integration user for OpenFn workflows, but it lacks concrete, platform-specific guidance. This issue tracks improving that section by cross-linking to existing Salesforce tips and adding new Google Service Account guidance.

Proposed changes

Salesforce — link to existing docs

The Salesforce adaptor page already covers:

• Recommending a dedicated "openfn" integration user
• Using the Salesforce API-only user license
• A link to Salesforce's official documentation on configuring this

Action: Add a reference/link from the Credentials page integration user section pointing to the Salesforce adaptor page for platform-specific guidance, rather than duplicating content.

Google — new Service Account guidance

The Google adaptors (Sheets, Drive, Healthcare, etc.) currently have no equivalent integration user guidance.

Action: Add a Google Service Account sub-section, covering:

• Recommend using a Google Service Account instead of a personal user OAuth credential for automated workflows.
• Service accounts are designed for machine-to-machine access and do not require a human login.
• The service account should be granted only the scopes/permissions required for the specific workflow (e.g., read-only access to Sheets if the workflow only reads data), following the principle of least privilege.
• Link to Google's documentation on creating and managing service accounts.

This guidance could live either on the Credentials page and/or the relevant Google adaptor pages (Sheets, Drive, etc.), consistent with how the Salesforce guidance is structured.