[FEATURE] Encapsulate "tx_ter_extensionkeys" access
A new object "ExtensionKey" now allows to work with Extension Keys in the SOAP API with an object-oriented approach.
This heavily reduces the tx_ter_helper class and removes helper functionality from tx_ter_api.
The new ApiUser is now used to validate requests that need a valid user (registering an API key, or transferring ownership) as argument.