Return lazy promises from steps instead of concrete values

Return promises of values instead of values, so once steps that depend on a value have run, the values aren't loaded when evaluating later steps

```ts
const lazyEmbedding = await step.runAction(..., { lazy: true });

const foo = await step.runMutation(api.foo.bar, { embedding: lazyEmbedding });
```
the args are now `LazyValue` where `LazyValue` is either a `Value` or `Promise` for primitives, or `LazyValue[]` or `Record` or `{ [K in keyof T]: T[K] | Promise }`