[Feature] Add ED25519 support for DKIM signing with OpenSSL version checks

[Feature] Add ED25519 support for DKIM signing with OpenSSL version checks

This commit adds support for ED25519 DKIM signatures when OpenSSL 1.1.1+ is available.
Key changes:

- Added HAVE_ED25519 detection in CMake to check for EVP_PKEY_ED25519 support
- All ED25519-specific code is conditionally compiled based on HAVE_ED25519
- When ED25519 is not supported, informative error messages are returned
- ED25519 keys loaded from PEM files are extracted and converted to libsodium format
- Fixed union handling to prevent double-free issues
- Updated tests to dynamically select key type based on request header
- Removed unused dkim-ed25519-pem.conf (cannot be passed via rspamc)

The implementation gracefully degrades on older OpenSSL versions while maintaining
full functionality when ED25519 support is available.