Automatically get the altool password from the MacOS keychain

[apyrgio]

During DMG notarization, we need an application password for altool. This password can be retrieved if one opens the Keychain GUI and copy pastes it to the prompt. To avoid this interaction though, we can make altool grab it directly from the keychain with -p "keychain:altool, provided we have saved it under that name.

This approach has the benefit that the user will be prompted to enter their login password once, or not at all, depending on when they logged in.