Add FindLibAssuan and FindLibGpgError

Open Ingo Klöcker requested to merge work/kloecker/find-libassuan-and-libgpg-error into master

(Copies of) these find modules are used by Kleopatra and will most likely also be used by libkleo.

I'm not sure about adding ws2_32 (a Windows library) to the INTERFACE_LINK_LIBRARIES of the LibAssuan::LibAssuan target. FindOpenSSL only adds it when linking statically, but it's also needed when linking dynamically.

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