Add amount field to rewards/commission response types

[joe-bowman]

Summary

Responses to reward withdrawal txs (MsgWithdrawDelegatorRewardResponse and MsgWithdrawValidatorCommissionResponse) do not contain information about the amount withdrawn.

Problem Definition

When rewards are withdrawn, an event is emitted containing the value of the withdrawn rewards (or comission), but the return types do not convey the same information.

When it comes to running said transactions via Interchain Accounts there is no way (without and additional cross chain lookup) to fetch this information, which is horribly inefficient.

Proposal

An amount field is added to the MsgWithdrawDelegatorRewardResponse and MsgWithdrawValidatorCommissionResponse types, and are populated in x/distribution/keeper/msg_server.go with the amount returned by each txs withdraw.

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