Implement a proxy backend to access private Serverless functions

Description

As a user, I want to invoke a Serverless function from the developer console. This action should be available as a page and as a modal.

Acceptance Criteria

1. A backend proxy to invoke a serverless function (or a k8s service in general) from the frontend without a public route.
2. The API endpoint should be only accessible to logged-in users.
3. Should also work when the bridge is running off-cluster (as developers start them mostly for local development)

Additional Details:

This will be similar to the web terminal proxy, except that no auth headers will be passed to the underlying service.

We need something similar to:

POST /proxy/in-cluster

{
  endpoint: string
  # Or just service: string ?? tbd.

  headers: Record<string, string | string[]>
  body: string
  timeout: number
}