Provide a way to capture addons purchases against Lifetime purchases. 

ex: Pabbly LTD account - 100$ addon 29$ to capture additional purchases. 

1. We will add a button inside of the sub screen "Addon"
2. This addon when pressed will be copy the existing details of the sub and creating a new one. 
3. The new addon will reference this sub and can be a one time or recurring in nature. 
4. Introduce new field "Sub_addon" and another field to capture the reference sub id. 
5. When sub is deleted, Addons will be also deleted.
6. When sub is Cancelled, Addons will be also cancelled, addons can be deleted or cancelled individually. 
7.  Add a new Label "Addon" to show for the subs. 
8. An LTD can have a Sub addons and Vice Versa. 
9. Adapt import/export feature with this addon feature. 
10. Combine this feature with https://app.productstash.io/subshero?card=61da653e49fcc4002a4be5cc