Support custom fields with HostedPages in API v4

[mpetrik-phrase]

What problem does your feature request address?

API v3 allowed something like this:

CheckoutNewForItemsRequest newForItemsRequest = HostedPage.checkoutNewForItems();
newForItemsRequest.param(MY_CUSTOM_FIELD_NAME, MY_CUSTOM_VALUE);

// where String MY_CUSTOM_FIELD_NAME = "subscription[cf_custom_field]";

However, there is no such option in API v4 - not generally or on the Subscription object level.

Describe the desired solution

I would appreciate having the same features for custom fields available in API v4 as well.

Alternatives considered

Not migrating this Hosted pages related API as the custom fields are needed for our business case.

Additional context

No response