Implement decryption for macOS keychain keys

[indygreg]

We have support for performing cryptographic signing using keys stored in macOS keychains. But we don't have support for decryption (needed for remote signing) because the Rust bindings to SecurityFramework.framework don't appear to have the APIs we need.

We'll likely need to teach the security-framework crate about the missing APIs in order to implement decryption.

As a workaround, you can export private keys from keychain to a PFX/.p12 file. This is probably less secure. But it will unblock using remote code signing with the keys.