Make JSONMarshaler methods require proto.Message

[aaronc]

Summary

JSONMarshaler Marshal/UnmarshalJSON methods should take proto.Message instead of interface{}. The lack of this is causing runtime errors specifically with non-pointer versions of proto types (GenesisState vs `*GenesisState).

Problem Definition

When we first started the protobuf migration, we had use cases for JSONMarshaler for types that we never intended to fully migrate to protobuf such as legacy REST requests/responses. HybridCodec was created along those lines of reasoning and all of the JSONMarshaler methods accept interface{} and do not require proto.Message.

However, proto JSON marshaling will fail for any type that does implement proto.Message. This is especially problematic with pointer types. *GenesisState does implement proto.Message but GenesisState does not and with the current JSONMarshaler this will only show up at runtime and this is problematic.

Proposal

1. Make JSONMarshaler methods require proto.Message. Amino JSON marshaling will still be possible but this ensures proto json marshaling doesn't unexpectedly fail at runtime.
2. Use *codec.LegacyAmino directly anywhere we specifically just want to use amino - these are the legacy queriers and REST endpoints. That code will need to be refactored to not use JSONMarshaler at all and just be amino only and eventually deprecated altogether.
3. *codec.LegacyAmino should not implement JSONMarshaler. Its MarshalJSON* methods should still accept interface{}. Where we need to use amino we use *codec.LegacyAmino. Were we want a choice between proto and amino we use JSONMarshaler

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