ICS2: Combine Header and Misbehaviour under ClientMessage interface

[colin-axner]

In ibc-go, we have decided to refactor the 02-client submodule along with the interfaces defined within the ClientState. One of the major changes we are doing is merging the Misbehaviour, Header types under a new interface ClientMessage. This allows light client implementations more flexibility to create other types that allow for client updates such as batch updates.

One motivator for this decision is described in #693

references

discussion leading to design
ibc-go issue