x/supply: Implement Querier

[alessio]

Description

As A an SDK REST/CLI user
I Want to execute queries against the supply module
So That I can learn about the network's supply related information

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[alexanderbez]

These need to be routed under /bank.

[alessio]

@alexanderbez please see #3972 (comment) - cc'ing @rigelrozanski

[rigelrozanski]

Yeah I guess this makes sense to be under bank.

[alexanderbez]

I don't think so anymore @rigelrozanski? We simply need a supply module querier.

[rigelrozanski]

Sure I mean from a users perspective I could see it being fine under bank or supply - keeping it in supply would make it more true to the code, but organizationally introduce a bit more noise in the CLI - but I don't really have a preference