vici: investigate use of generics in API

[enr0n]

With go 1.18 coming, it would be good to investigate the use of generics in the vici API. On area that may be useful is around the Message type, since we currently use a map[string]interface{} under the hood. Using generics in the vici package would introduce breaking changes, which is why this is an issue targeted for a future v1.0 tag.