Interactive UI for Custom API Management in Rill Web

[Kavinjsir]

Problem: Currently, creating a custom API in Rill involves manually writing a YAML file, which can be error-prone and lacks immediate feedback. Specifically, it's challenging to:

1. Verify the syntax before deployment.
2. View the results of API calls.

Proposed Solution: As a Rill user, I want an interactive UI in Rill Web for managing custom APIs, enabling me to:

1. Use localhost:9009 (started with rill start .) to access a UI for configuring custom APIs and receiving immediate feedback based on my input. This should be similar to the existing UI for creating models and dashboards.
2. View a "Custom APIs" tab in Rill Cloud after deployment, displaying the list of custom APIs, their status, and ideally, some sample results. This would provide a consistent and seamless experience from defining sources to designing models and creating custom APIs.