Define fee on REST txs

[fedekunze]

Summary

Add fee to baseReq on txs POST body for users to define their own fee value.

Problem Definition

There's no way to define fees now on the REST txs

Proposal

Add a Fee property to the BaseReq:

cosmos-sdk/client/utils/rest.go

Lines 121 to 131 in 03771a3

	// BaseReq defines a structure that can be embedded in other request structures
	// that all share common "base" fields.
	type BaseReq struct {
	Name string `json:"name"`
	Password string `json:"password"`
	ChainID string `json:"chain_id"`
	AccountNumber uint64 `json:"account_number"`
	Sequence uint64 `json:"sequence"`
	Gas string `json:"gas"`
	GasAdjustment string `json:"gas_adjustment"`
	}

cc: @jackzampolin @alessio @faboweb @alexanderbez

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