👋 Thanks for creating a PR!

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.

For contributor use:

• 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

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

Thank you for your contribution to the Cosmos-SDK! 🚀

Codecov Report

Merging #6151 into master will increase coverage by 0.04%.
The diff coverage is 68.65%.

@@            Coverage Diff             @@
##           master    #6151      +/-   ##
==========================================
+ Coverage   54.67%   54.72%   +0.04%     
==========================================
  Files         441      443       +2     
  Lines       26592    26703     +111     
==========================================
+ Hits        14539    14612      +73     
- Misses      11034    11052      +18     
- Partials     1019     1039      +20     

Yeah, so a few things make it complex:

• in order to marshal/unmarshal amino interfaces we need an actual amino codec instance
• the Marshal/UnmarshalAmino and Marshal/UnmarshalJSON methods don't take any sort of context value that we could overload to pass through an amino codec
• amino unmarshal actually requires an interface pointer to be passed to it (i.e. you can't just pass interface{} you need to pass something registered like sdk.Msg), so doing this via reflection would be hard

So this solution basically takes advantage of the fact that the UnpackInterface methods we're using for Any, pass a codec-like thing around that expects an interface pointer just like amino expects. So turned out I could overload that functionality to make Anys work with amino without any hacky things like a global amino or gls and without needing a separate set of amino-only types.

Does that help explain things at all?