WIP: Add PLUS AccountType
This commit adds support fo a new subscription called PLUS.
That subscription is created when an existing shop user migrates his account to a LDH one.
We have 2 ways of creating the new PLUS subscription, through the middleware using REST API or through the WC itself, using Purser plugin (triggered through by the middleware in WC_UpgradeView
).
This commit uses the second approach. The first approach is commented in the code.