Implement `ctx.IsPrepareProposal` and `ctx.IsProcessProposal`

[rootulp]

Motivation

Discussion w/ @evan-forbes

Context

The SDK ctx has these methods:

https://github.com/celestiaorg/cosmos-sdk/blob/8c98dec8632ee91701b5676530c90292a6c193ee/types/context.go#L60-L61

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.

[tac0turtle]

this is done with v50, can we close this

[rootulp]

Where? I don't see it here.

[tac0turtle]

sorry i misread. i thought it was a downsteaming issue but it seems like a new feature