Release notarized macOS binaries

[maryla-uc]

Currently, libavif releases are set up to include precompiled binaries for Windows, Linux and macOS (coming soon).
However, for security reasons, naively built macOS binaries will not run and will display a popup like "“avifenc” can’t be opened because Apple cannot check it for malicious software."
Users must manually remove the quarantine bit with sudo xattr -r -d com.apple.quarantine
To do things properly, the release binaries should be signed and notarized. See https://developer.apple.com/documentation/xcode/notarizing_macos_software_before_distribution