Make IBCtesting GetStakingKeeper be based on an interface

[ValarDragon]

Summary

The IBCtesting GetStakingKeeper required interface method (https://github.com/cosmos/ibc-go/blob/main/testing/app.go#L39) should not depend on the literal staking keeper in the hub.

This should instead have an interface dependency, to account for apps doing weirder things with their staking modules. E.g. Osmosis superfluid staking, we really need to look at what IBC depends on, to determine what we should be returning.

Also would recommend renaming to GetIBCTestingStakingKeeper as part of this.

Problem Definition

The current design inhibits modularity of ibctesting, for a small reason.

Proposal

Define an interface for what keeper methods IBC depends on from the staking keeper, and then define an interface accordingly. Use that interface in the required ibctesting app interface.

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[colin-axner]

@ValarDragon if the API is changed to:

GetStakingKeeper() ibctestingtypes.StakingKeeper

do you still want this change:

Also would recommend renaming to GetIBCTestingStakingKeeper as part of this.

still the return value implies it is a test staking keeper