Openssl dependency cleanup

[RichLogan]

• Move OpenSSL dependency directly to libhpke definition.
• Replace add_compile_definitions with target_compile_definitions to ensure proper propagation of defines.
• Added a test to ensure the custom WITH_BORINGSSL define gets set when compiling with BoringSSL - maybe overkill but helped me validate this change so might be useful. I had to set a property on test discovery to make ctest not see skips as failures. Ref: CTest (using catch_discover_tests) treats skipped tests as failed catchorg/Catch2#2873

Although this is a standalone change the motivation here was to do some pre-work for looking into #425