impr(erc20): Add Informal Audit IF-ETHERMINT-06 protection

[danburck]

Description

Closes: https://linear.app/tharsis/issue/ENG-376/if-ethermint-06-direct-balance-manipulation

This PR adds account balance checks for the coin and token conversions in order to prevent malicious contracts to change the balances differently than expected.

[linear]

ENG-376 IF-ETHERMINT-06: Direct balance manipulation

• Add a check that the expected balances after executing transfer match the actual balances via runtime monitoring
• Update spec invariants

[codecov]

Codecov Report

Merging #191 (d69e1e0) into main (71c4413) will decrease coverage by 1.91%.
The diff coverage is 43.67%.

❗ Current head d69e1e0 differs from pull request most recent head 53892a1. Consider uploading reports for the commit 53892a1 to get more accurate results

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
- Coverage   72.78%   70.86%   -1.92%     
==========================================
  Files          33       32       -1     
  Lines        2065     2063       -2     
==========================================
- Hits         1503     1462      -41     
- Misses        499      529      +30     
- Partials       63       72       +9     

Impacted Files	Coverage Δ
x/erc20/keeper/keeper.go	100.00% <ø> (ø)
x/erc20/keeper/msg_server.go	68.49% <38.75%> (-13.28%)	⬇️
app/app.go	80.58% <100.00%> (-0.09%)	⬇️
x/erc20/types/params.go	100.00% <100.00%> (ø)

[fedekunze]

Few changes requested. Also if you can update the function's comments and spec to reflect the new checks 🙏

[fedekunze]

ACK pending changelog entry 👍