Add upgrade handlers to simapp

[anilcse]

Summary

While working on integration tests as part of QA, observed that there is no clean way to automate software upgrade tests. Having upgrade handlers for each version would make it easy for automating this and thus improve test coverage. It can also act as an example for teams trying to build upgrade handlers

Proposal

Add upgrade handlers for simapp for each version.

v045 contains the following upgrade handler:

• upgrade-v044-to-v045

Similarly v0.46.x will have,

• upgrade-v045-to-v046

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[alexanderbez]

I like the idea

[ValarDragon]

I'm in favor of this!

I'd actually like to suggest changing the simapp app.go code structure to be like Osmosis'. (Especially after osmosis-labs/osmosis#1379 )

Namely, using a common Upgrade and Fork struct, and making a separately defined struct for all the module keepers. I think the package structure used there, relative to gaia, makes the package much more scalable. (And helps mitigates the footguns currently present around not using pointers for all keepers wrt hooks)

[tac0turtle]

done here https://github.com/cosmos/cosmos-sdk/blob/main/simapp/upgrades.go, closing, but if there is more please open a new issue