imp(evm): define minimum GasUsed proportional to GasLimit (#1087)

* min gas denominator implementation

* update changelog

* modify MinGasDenominator type to sdk.Dec

* fix typo in comments

* add comments

* update comment

* refactor logic

* remove unnecesary test

* fix typo on proto

* rename param

* fix tests

* use truncate and run mod tidy

* comment to default value

* update changelog

* rename temporary gas used

* integration tests

* add migrations

* add default as var

Co-authored-by: Federico Kunze Küllmer <31522760+fedekunze@users.noreply.github.com>

proto/ethermint/evm/v1/evm.proto

@@ -24,6 +24,12 @@ message Params {
     (gogoproto.moretags) = "yaml:\"chain_config\"",
     (gogoproto.nullable) = false
   ];
+  // min gas denominator bounds the minimum gasUsed to be charged
+  // to senders based on GasLimit
+  string min_gas_multiplier = 6 [
+    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Dec",
+    (gogoproto.nullable) = false
+  ];
 }
 
 // ChainConfig defines the Ethereum ChainConfig parameters using *sdk.Int values