event context and http middleware

[djgrant]

Adds support for context in workflows and HTTP middleware.

Currently middleware can be attached to the HTTP server.

const middleware = createMiddleware(async (req, event, next, logger) => {
  // Before workflow invocation
  event.context.set('key', 'value');
  
  // Continue to next middleware
  const response = await next();
  
  // After workflow invocation
  response.headers.set('X-Custom-Header', 'value');
  return response;
});

const server = createWorkflowHttpServer({
  port: 8080,
  logger,
  invoker,
  middleware: [middleware]
});

When a workflow is invoked, the event object now includes context.

import { workflow } from "yieldstar";

export const simpleWorkflow = workflow(async function* (step, event, logger) {
  const key = event.context.get('key');
  yield* step.delay(1000);
  return key;
});

Mutations to context during workflow phases are not persisted.

Todo:

• disable context.set to make prevent context being mutated in workflows
• throw an error from middleware if context is mutated after calling next