Support custom MessageHandler in wasm

[alpe]

An interface would be a great extension point to the system.

We provide a default implementation but people may want to use a different implementation in their system or for testing.
This is only a small change in keeper

[alpe]

As an implementation detail:
We could have the following type definition

// Option is an extension point to instantiate keeper with non default values
type Option interface {
	apply(*Keeper)
}

With implementations that can set new values on private attributes. The constructor would be easily extendable:

func NewKeeper(
	cdc codec.Marshaler,
	[...]
	opts ...Option,
) Keeper {
// and apply them all
	for _, o := range opts {
		o.apply(&keeper)
	}

[ethanfrey]

I think this was closed in the IBC mega-merge. @alpe can you verify and close if true?

[alpe]

Hot done for MessageHandler, yet