Pass through an AppIniter func to be able to launch chains with different apps

[danwt]

Issues

Partially tackles

• Create example of app.go for users, test in real configuration, fix IBC-Go test suite to handle it cosmos/interchain-security#58

Related to

• Create minimum runnable consumer chain cosmos/interchain-security#59
• Restructure Interchain Security repo for release cosmos/interchain-security#77

Overview

There is a goal to split provider/consumer apps so that people can build from a minimal viable consumer chain. This PR is paired with another PR in cosmos/interchain-security. The two PR's together

1. Split the app.go into two app.go's. One for parent and child.
2. Modify ibc-go testing to be able to use two different app.go's.

Therefore this PR should be merged together with

• Support different app.go's for consumer and provider cosmos/interchain-security#84

---

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• Targeted PR against correct branch (see CONTRIBUTING.md)
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• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[danwt]

@jtremback - we said earlier to fork the testing parts of ibc-go into a separate repo and make changes there. That can still be the plan, but for now I have just changed informalsystems/ibc-go

[sainoe]

LGTM. Great work @danwt !