(toolkit-lib): Public message codes enum

Providing access to the currently private https://github.com/aws/aws-cdk-cli/blob/main/packages/%40aws-cdk/toolkit-lib/lib/api/io/private/messages.ts#L527 IO and SPAN enums, which would allow us to extract this info from the IoHost messages without having to hardcode the various IO codes (like CDK_TOOLKIT_I4001) in the host code.