User Interface instead of *baseapp.MsgServiceRouter in modules

[alpe]

Summary

There is a tight coupling when the *baseapp.MsgServiceRouter type is expected in the modules. In order to make this more extendable and future proof an interface would be better.

Proposal

ibc-go and wasmd modules are using interfaces already.

// MessageRouter ADR 031 request type routing
// https://github.com/cosmos/cosmos-sdk/blob/main/docs/architecture/adr-031-msg-service.md
type MessageRouter interface {
	Handler(msg sdk.Msg) baseapp.MsgServiceHandler
}

[tac0turtle]

created a pr, in the near future this wont be needed anymore with things like adr38