Add a `PromisableFields` type

[stabai]

Facade objects often need to be constructed by pulling data from multiple asynchronous calls. It would be nice if there were a way to create an object that can contain Promisable<> versions of its fields:

type PromisableFields<ObjectType> = {
  [Key in keyof ObjectType]: Promisable<ObjectType[Key]>;
};

interface Foo {
  bar: number;
  baz: string;
}

type UnresolvedFoo = PromisableFields<Foo>;
/*
This would yield the following type:
{
  bar: Promisable<number>;
  baz: Promisable<string>;
}
*/

The user would need a function to reconstruct the object after resolving promises, but this would be pretty simple:

export async function resolveFields<ObjectType>(
  promisableObject: PromisableFields<ObjectType>
): Promise<ObjectType> {
  const partial = {} as Partial<ObjectType>;
  for (const [key, promisableValue] of Object.entries(promisableObject)) {
    partial[key] = await promisableValue;
  }
  return partial as ObjectType;
}

If this would be acceptable, I'm happy to create an PR with it and some tests.

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