chore(app) App refactor after PR #972, move initialization logic outside of the app and into keepers package

[zale144]

Description

In the PR #972 the upgrade handlers are refactored so that they use an interface for the upgrades themselves and keepers are extracted to be in a separate package.

Here we go a step further and move the initializing logic out of app.go and into the new package keepers.

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Closes #320

All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow-up issues.

PR review checkboxes:

I have...

• [ ] Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• [ ] Targeted PR against the correct branch
• [ ] included the correct type prefix in the PR title
• [ ] Linked to the GitHub issue with discussion and accepted design
• [ ] Targets only one GitHub issue
• [ ] Wrote unit and integration tests
• [ ] Wrote relevant migration scripts if necessary
• [ ] All CI checks have passed
• [ ] Added relevant godoc comments
• [ ] Updated the scripts for local run, e.g genesis_config_commands.sh if the PR changes parameters
• [ ] Add an issue in the e2e-tests repo if necessary

SDK Checklist

• [ ] Import/Export Genesis
• [ ] Registered Invariants
• [ ] Registered Events
• [ ] Updated openapi.yaml
• [ ] No usage of go map
• [ ] No usage of time.Now()
• [ ] Used fixed point arithmetic and not float arithmetic
• [ ] Avoid panicking in Begin/End block as much as possible
• [ ] No unexpected math Overflow
• [ ] Used sendCoin and not SendCoins
• [ ] Out-of-block compute is bounded
• [ ] No serialized ID at the end of store keys
• [ ] UInt to byte conversion should use BigEndian

Full security checklist here

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For Reviewer:

• [ ] Confirmed the correct type prefix in the PR title
• [ ] Reviewers assigned
• [ ] Confirmed all author checklist items have been addressed

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After reviewer approval:

• [ ] In case the PR targets the main branch, PR should not be squash merge in order to keep meaningful git history.
• [ ] In case the PR targets a release branch, PR must be rebased.

[omritoptix]

@zale144 please give it a meaningful pr title. thanks.