Add issue template for "Module Readiness Checklist"

[clevinson]

Description

For Regen Ledger we've decided to adopt a formal checklist for performing internal audits of modules before publishing them live in a stable release.

It's been suggested (and also discussed w/ @aaronc & @shahankhatch) that it would be useful to adopt that same process for all new modules being merged into the SDK. This PR represents our proposal for such a checklist. We're happy to upgrade it with any suggestions from the wider group of Cosmos SDK core devs.

Rendered Preview.

Once this gets approved & merged, we'd like to kick off the process by going through a corresponding internal audit using this checklist for x/authz and x/fee_grant modules.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• 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

[alexanderbez]

Should we also require some base level minimum test coverage? e.g. at least 90% of the state-machine is covered and 70% of the client code?

[orijbot]

Benchmark beginning. Status page: https://github.orijtech.com/benchmark/status?commit=1a80194f277f1931ff33f069f0dc2bf1a5367e03