bug(erc20): registerCoin deploys erc20 contract with different minimal coin denomination

[danburck]

Description

Closes: https://linear.app/tharsis/issue/ENG-351/registercoin-deploys-erc20-contract-with-different-minimal-coin

This PR makes sure that the deployed erc20 has the same decimals as the coin during registerCoin.

[linear]

ENG-351 `registerCoin` deploys erc20 contract with different minimal coin denomination

When you register a coin right now with decimals != 18 it will have a different decimal for the deployed ERC20 token. This can be very confusing for users as the coin and the token might use different prefixes (e.g. aPhoton). The deployed ERC20 Token has a standard of 18 decimals hardcoded.

To avoid confusion, we need to create a contract that overrides the decimals on deployment.

[codecov]

Codecov Report

Merging #200 (bca5042) into main (350a9ad) will increase coverage by 0.10%.
The diff coverage is 100.00%.

❗ Current head bca5042 differs from pull request most recent head 6f7d210. Consider uploading reports for the commit 6f7d210 to get more accurate results

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
+ Coverage   72.04%   72.15%   +0.10%     
==========================================
  Files          32       32              
  Lines        2168     2176       +8     
==========================================
+ Hits         1562     1570       +8     
  Misses        534      534              
  Partials       72       72              

Impacted Files	Coverage Δ
x/erc20/keeper/msg_server.go	69.83% <100.00%> (+0.20%)	⬆️
x/erc20/keeper/proposals.go	91.17% <100.00%> (+0.26%)	⬆️

[fedekunze]

ACK! Can you add a bug fix changelog entry too?