Provide libappimageupdate C interface for FFIs

[Korne127]

Currently, libappimageupdate is only available as a C++11 shared library. However, many other programming languages only provide foreign function interfaces (FFIs) only for C code, which serves as the protocol for inter - programming language communication.
While it's possible to write a custom wrapper or a C++ FFI tool, it would be much easier to use this libraries in other programming languages if there was a C interface (created manually or with something like SWIG) that also provided access to the Updater class and its methods.