Add proto3 json as ICS 27 encoding type

[colin-axner]

See discussion on cosmos/ibc-go#3246 for justification of the addition.

Related issue: #634

[Teja2045]

@colin-axner, is this still open? I think cosmos/ibc-go#3796 should close this. Or do let me know if anything else needs to be done.

[colin-axner]

Yes I believe it is still open unless the specs in this repo have been updated?

[crodriguezvega]

Wondering if we need to add this to spec... Currently the spec doesn't really specify any possible encoding formats and seems to leave this to the implementations... what do you think, @AdityaSripal?

[colin-axner]

It's not possible to integrate correctly these applications without knowing the expected encoding. My comment here is an example where using proto3 json vs standard json is not compatible

It is a problem that the spec doesn't specify encoding formats