These types are extracted out of Vitess source and only modified to be more friendly for others.
The main interface, besides MySQL, for Vitess is via gRPC. gRPC uses protobuf. If a third party module wanted to interact with Vitess's gRPC APIs, or to just work with their types such as a QueryResult, the logical choices is to use the generated interfaces from vitess.io/vitess/go/vt/proto, but this dependency is quite large since this is the entirety of Vitess.
So this extraction is an attempt at being a much lighter-weight version that is fully capable of interoping with Vitess. This makes it more friendly for consumers to wrap and depend on Vitess types, without a dependency on Vitess.