Add amino compatibility layer for proto Any

[aaronc]

This PR adds backwards compatibility for Amino binary and JSON encoding for structs that choose to encode interfaces with google.protobuf.Any instead of plain go interfaces.

Structs that follow the guidelines laid out in #6081 (implementation of UnpackInterfaceMessages) should have identical Amino binary and JSON encoding as the previous structs (without Any) if AminoCodec is used to marshal/unmarshal them.

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[codecov]

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     

[fedekunze]

utACK

[alexanderbez]

utACK. To be honest, and it could very well be just me, but I find this level of indirection of codec passing/usage extremely confusing. I had to go back and forth between files/types numerous times to really see what's happening.

But as long as it works (which it seems to) and the user-level API remains clean and minimal, that is the important part -- I doubt this code will change much.

[aaronc]

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?