Add and expose custom customer data to cart

Adding custom data for the customer to the cart at the moment is added as meta data.

However, some users have asked for the data to be exposed without the need of using a checkout field editor or having to filter in the additional fields first.

Much like how the cart fees are captured so they can be calculated later, a similar system is needed in order to proceed with this request and expose those additional fields.

How do we determine which fields are to be exposed?

Just have to experiment for now on which method is best.

There is however a down side to this when the cart session is loaded to the native checkout, those fields will not show and the customer will not be able to update them there.