Set default value for flow and third party flows

What does this MR do and why?

• Adds default values to Custom Flows and External agents (a.k.a Third-party flows)
• Updates order of required fields in description to match skeleton

Adds follow up issue based on ideas in this MR: #580069

References

• Initial discussion in issue
• Final YML string and follow up ideas

Screenshots or screen recordings

Custom Flow	External agent

How to set up and validate locally

1. Prerequisite: You have a Duo workflow-enabled project.
2. Enable the feature flags for the global_ai_catalog and flows, and third_party_flows (Product decisions)
3. Visit http://gdk.test:3000/explore/ai-catalog/flows/new
4. Toggle between Flow and Third-party Flow

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #578228 (closed)