Fee middleware: query endpoints

[seantking]

Summary

The fee module should also expose the following queries so that relayers may query their expected fee:

// Gets the fee expected for submitting ReceivePacket msg for the given packet
// Caller should provide the intended relayer address in case the fee is dependent on specific relayer(s).
function GetReceiveFee(portID, channelID, sequence, relayer) Fee

// Gets the fee expected for submitting AcknowledgePacket msg for the given packet
// Caller should provide the intended relayer address in case the fee is dependent on specific relayer(s).
function GetAckFee(portID, channelID, sequence, relayer) Fee

// Gets the fee expected for submitting TimeoutPacket msg for the given packet
// Caller should provide the intended relayer address in case the fee is dependent on specific relayer(s).
function GetTimeoutFee(portID, channelID, sequence, relayer) Fee

Problem Definition

Write GRPC endpoints for the queries defined in query.proto & associated getter keeper functions.

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