Implement ctx.IsPrepareProposal and ctx.IsProcessProposal

[rootulp]

Motivation

Discussion w/ @evan-forbes

Context

The SDK ctx has these methods:

cosmos-sdk/types/context.go

Lines 60 to 61 in 8c98dec

	func (c Context) IsCheckTx() bool { return c.checkTx }
	func (c Context) IsReCheckTx() bool { return c.recheckTx }

Problem

The SDK ctx doesn't have methods to determine if the ctx is inside PrepareProposal or ProcessProposal

Proposal

Add methods for IsPrepareProposal and IsProcessProposal.