Integration Support: REST API, Editor, Example Browser

[satchelbaldwin]

This PR adds support and documentation for Integration Providers, Integrations, associated resources on Integrations, as well as REST API routes for accessing those providers.

Integration Providers, stored on the context, provide a list of implementation-defined Integrations to extend the agent with support for everything from external datasources, local datasets, and, in the future, having support ready for adding agent-to-agent or MCP connections.

This change moves integrations from being provided mostly through a second library to an extensible first class system inside Beaker.

Adds:

• Documentation files for integrations and how they work -- concepts and for custom provider implementations
• Integration side panel, listing the available datasources
• Scaffolding for a future plugin system
• Editing examples for integrations via the editor page or through the add example tool
• Typing around integrations and resources
• Docker build fixes