Dynamic fields

[chrisvxd]

Similar to resolveData, we should consider a resolveFields method to allow users to dynamically set their fields based on the current props.

Alternatively, we could piggy-back off resolveData and allow it to return a dynamic fields type.

This may also resolve #188.

---

Edit: Showing / hiding fields is possible using field overrides, which provide a suitable intermediary solution for some use cases.

Example that hides the image object field on the demo Hero component when the align property equals "center":

<Puck
  // ...
  overrides={{
    fieldTypes: {
      object: ({ children, name }) => {
        const { selectedItem } = usePuck();

        if (selectedItem.type === "Hero") {
          if (
            selectedItem.props.align === "center" &&
            name === "image"
          ) {
            return null;
          }
        }

        return <>{children}</>;
      },
    },
  }}
/>