refactor(js/flows): made flow be actually actions, actions can stream, moved flow server to express plugin

[pavelgj]

Removed all express stuff from flows core. Flows are just simple actions now. Actions have the flow interface (including streaming).

Removed auth policy from flows. The policy is now a plugin feature. startFlowServer also moved to the express plugin.

import { startFlowServer, withAuth } from '@genkit-ai/express';

startFlowServer({
  flows: [
    simpleFlow,
    withAuth(flowThatNeedsAuth, authProvider, ({ auth, action, input, request }) => {
      if (!auth) {
        throw new Error('Authorization required.');
      }
    })
  ],
});

run is now ai.run.

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)

[chrisraygill]

Remind me, what's the use case for startFlowServer since we introduced the Express handler?

[pavelgj]

Remind me, what's the use case for startFlowServer since we introduced the Express handler?

It much less boilerplate for simple cases. And we have existing users.... I don't want to break them. So I'm moving it rather than removing.