This adds pre-built OpenSSL binaries to the Windows 3rd-party deps. The binaries are provided by the curl project   using a reproducible process with publicly-viewable build logs. It should be reputable enough for our usage.
Note: These binaries are code-signed by a self-signed certificate, therefore the build on the binary-factory will end up resigning them with KDE's certificate.
I've took a look at building OpenSSL, but the officially supported way is to build with MSVC and the other tested methods involve either MSYS2 or Cygwin. I didn't know how difficult would be to adapt it for a native mingw-w64 build process so I hadn't tried to do it.
Note for @rempt - If this revision should be accepted, please upload the two archives to KDE's files server so I can use them in the final commit.