feat(cli,js): implemented reflection API v2 in CLI and JS

[pavelgj]

it's behind --experimental-reflection-v2 flag.

RFC: #4211

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello @pavelgj, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request implements the experimental Reflection API V2, which shifts from HTTP to WebSockets for improved communication between the Genkit CLI and Genkit Runtimes. This architecture simplifies runtime management and introduces streaming capabilities. The changes include a new WebSocket-based runtime manager, updates to the CLI start command, and modifications to the common library to support the new protocol.

Highlights

• Reflection API V2 Implementation: This PR introduces an experimental implementation of Reflection API V2, utilizing WebSockets for communication between the Genkit CLI and Genkit Runtimes. It's enabled via the --experimental-reflection-v2 flag.
• WebSocket-Based Communication: V2 reverses the connection direction compared to V1, with the Genkit CLI starting a WebSocket server and the Genkit Runtime connecting as a client.
• Streaming Support: The PR extends JSON-RPC 2.0 to support streaming using Notifications for streamChunk and runActionState messages.
• CLI Changes: The start command in the CLI is modified to include options for enabling the experimental reflection server and specifying allowed telemetry CORS hostnames.

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[gemini-code-assist]

Code Review

This pull request introduces Reflection API V2, which leverages WebSockets for bidirectional communication between the Genkit CLI and Runtimes. This is a significant architectural change that reverses the connection direction, allowing the CLI to act as a server and Runtimes as clients. The implementation includes a new BaseRuntimeManager abstract class, with separate concrete implementations for V1 (HTTP-based) and V2 (WebSocket-based) managers, ensuring a clean separation of concerns. The changes also include necessary dependency updates, new CLI options to enable V2, and comprehensive test coverage for the new WebSocket manager and server components. The documentation for the V2 protocol is well-detailed, outlining the message formats and protocol flow. Overall, this is a well-designed and implemented feature that enhances the flexibility and capabilities of the Genkit reflection mechanism.