[Native Banker] Implement TotalCoin

[leohhhn]

Description

This issue concerns the implementation of the TotalCoin function in the native Gno banker. This function should return the total supply of a specific coin given its denomination.

Reference code can be found here.

[DIGIX666]

Hello !
I've tried to solve this problem with #2670
I hope I had the right approach :)

[deelawn]

I'm not sure if we want this. I just took a look how ERC-20 tokens work and the contract must implement the IERC20 interface that has a method called totalSupply() -- I kind of like the total coins issued being encapsulated in the token definition itself rather than having to rely on the protocol to maintain this information; it forces the author to bear the responsibility of any additional storage cost incurred, which is, in this case, the total token count value. It also gives the realm that mints the coin the autonomy to determine whether or not they want to expose the total token count. I'm can't think of a reason why they'd want to do this but maybe they would?

So here is what I'm getting at -- could we just rely on the existing Banker in the GRC20 package to manage all of this? If the realm that created the token wants to expose its read-only methods, they can. Or maybe we can implement better GRC20 banker security so that the owning realm can export it with the assurance that calling modifying methods can only be done by the users authorized to do so -- then anyone could import the realm owning the token and read all of its properties.

I've changed my mind on this after a few discussions.