feat(mint): Implement custom inflation function. (cosmos#10441)

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## Description

We want to use this feature in [Akash Network](https://akash.network/).

The [Akash whitepaper](https://whitepaper.io/document/632/akash-network-whitepaper) defines a time-based function for the inflation: `I(t)`. The way `cosmos-sdk` currently calculates inflation is different as compared to what the Akash whitepaper prescribes. So, we need the `cosmos-sdk` to accept a custom function to calculate inflation. When the custom function isn't provided, it should use the default inflation calculation logic as it currently does.

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---

### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

app.go

@@ -332,7 +332,7 @@ func NewSimApp(
 		crisis.NewAppModule(&app.CrisisKeeper, skipGenesisInvariants),
 		feegrantmodule.NewAppModule(appCodec, app.AccountKeeper, app.BankKeeper, app.FeeGrantKeeper, app.interfaceRegistry),
 		gov.NewAppModule(appCodec, app.GovKeeper, app.AccountKeeper, app.BankKeeper),
-		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper),
+		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper, nil),
 		slashing.NewAppModule(appCodec, app.SlashingKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),
 		distr.NewAppModule(appCodec, app.DistrKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),
 		staking.NewAppModule(appCodec, app.StakingKeeper, app.AccountKeeper, app.BankKeeper),
@@ -384,7 +384,7 @@ func NewSimApp(
 		capability.NewAppModule(appCodec, *app.CapabilityKeeper),
 		feegrantmodule.NewAppModule(appCodec, app.AccountKeeper, app.BankKeeper, app.FeeGrantKeeper, app.interfaceRegistry),
 		gov.NewAppModule(appCodec, app.GovKeeper, app.AccountKeeper, app.BankKeeper),
-		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper),
+		mint.NewAppModule(appCodec, app.MintKeeper, app.AccountKeeper, nil),
 		staking.NewAppModule(appCodec, app.StakingKeeper, app.AccountKeeper, app.BankKeeper),
 		distr.NewAppModule(appCodec, app.DistrKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),
 		slashing.NewAppModule(appCodec, app.SlashingKeeper, app.AccountKeeper, app.BankKeeper, app.StakingKeeper),