Add support for locating ykcs11 library on macOS

[ye4241]

This pull request enhances the way the getYkcs11Library() method in YubiKey.java locates the YubiKey PKCS#11 library, making it more flexible and robust, especially for custom and MacOS environments.

Improvements to library discovery:

• Added support for specifying a custom PKCS#11 library path via the PKCS11_MODULE environment variable, allowing users to override the default search logic.
• Improved MacOS support by searching multiple common locations for libykcs11.dylib, including paths from the DYLD_LIBRARY_PATH environment variable, increasing compatibility with different installation methods.