Add root error info to public error

[tonidero]

Description

This is a possible approach to add root error info to public errors exposed by the SDK. Many times, we have nested errors, which makes discoverability of the root error difficult. This also hides actual StoreKit errors from being easily accessible, which some devs have asked about.

This PR adds a new property to the userInfo of the public error exposed by the SDK including relevant information about the root error. If the root error is a type of StoreKitError, we add some additional information related to that.

For example, when simulating an error getting products during the getOfferings call using StoreKit, a new property in userInfo rc_root_error will be added with the following structure:

{
    code = 4;
    domain = "StoreKit.StoreKitError";
    localizedDescription = "Item Unavailable";
    storeKitError =     {
        description = "not_available_in_storefront";
    };
}

[tonidero]

I want to test this more and add some tests next week, but I wanted to get some feedback, specially from iOS folks, to make sure this makes sense and if there are other ideas to expose the root StoreKit error if any to developers 🙏

[tonidero]

My main concern with this approach is whether we only expose PublicError everywhere and/or if we use this method somewhere else... Need to research it a bit more.