Make sdk.Msg implement proto.Message

[aaronc]

ref: #6213

This PR makes sdk.Msg inherit proto.Message. As a result all sdk.Msg types now must use pointer sematnics.

This is necessary for packing sdk.Msgs in proto Anys and in particular to complete #6216.

In some cases I forced certain Msg's to implement proto.Message with dummy implementations (in particular ibc/07-tendermint and a few test Msg's), but I tried to avoid this where possible and added a proto file for ibc/09-localhost MsgCreateClient.

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[codecov]

Codecov Report

Merging #6327 into master will decrease coverage by 0.03%.
The diff coverage is 69.51%.

@@            Coverage Diff             @@
##           master    #6327      +/-   ##
==========================================
- Coverage   55.69%   55.66%   -0.04%     
==========================================
  Files         448      448              
  Lines       26941    26956      +15     
==========================================
  Hits        15005    15005              
- Misses      10862    10877      +15     
  Partials     1074     1074              

[jaekwon]

It isn't clear why Msgs need to become pointer receivers everywhere. Msgs should stay immutable, and keeping them as non-pointers helps with that (though not all the way, as slices for example are still mutable, but regardless it helps). The Reset() function could still take a pointer receiver but everything else can and arguably should stay the same.

Pointer-fying also comes with baggage like needing to check for nils, so ideally we don't change anything that isn't already working.

What is the API for unmarshaling any, UnmarshalAny()? The above should work for this purpose.

[alexanderbez]

@jaekwon I believe specifically because:

This is necessary for packing sdk.Msgs in proto Anys

I advocated for clear semantics -- pointer or non-pointer only, don't mix. @aaronc brought to my attention that in order for a sdk.Msg to be packed into a tx's Any, it must obey proto.Message, which requires pointer semantics

[aaronc]

Protobuf code generation only supports pointer types. That is why the change is needed. In the future making changes to the code generation itself could be an option, but it isn't right now. So unfortunately l don't think the migration can proceed without this change.

[alexanderbez]

conceptACK