[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.
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