Add an API for composing workflows

[benjavicente]

export const parentWorkflow = workflow.define({
  handler: async (step) => {
    await step.runWorkflow(internal.example.childWorkflow, childWorkflowArgs)
  }
})

If the parent workflow is canceled, every child workflow should be canceled.

[benjavicente]

Alternative (and wilder) idea: have a promise-like-API

// The run is disconected from the parent workflow
const childWorkflowReference = await step.workflow.start(internal.example.childWorkflow, childWorkflowArgs)

const value = await step.workflow.wait(childWorkflowReference) // await
await step.workflow.allSettled( ... ) // returns the statues and values 

await step.workflow.all( ... )  // if any workflow faill, calcel the parent and every child that is running
await step.workflow.any( ... ) // get the first that works, cancel every other that is still running
await step.workflow.race( ... )  // get the first workflow that ended, cancel every other that is still running
// Since the last 3 don't wait for every workflow to stop, there could be a second argument like `{ cancelOthers: true }`