feemarket: global MinGasPrice parameter (#1104)

* Add min_gas_price to feemarket params

* Add MinGasPriceDecorators

* feemarket integration tests for MinGasPrice

* Restructure integration tests

* Simplify integration tests context

We use DeliverTx context to set up the app, otherwise not all settings are initialized.
We test CheckTx with `s.app.BaseApp.CheckTx(req)`, which uses the `CheckTx` mode and context.

* Update MinGasPrice spec in feemarket module

* reorder ethermint module order for initializing genesis

* feemarket migrations for adding MinGasPrice param

* update changelog

* Additional unit tests for MinGasPrice = 0, tx gas price > 0 (PR review)

https://github.com/tharsis/ethermint/pull/1104#discussion_r884991661

* Use 0 MinGasPrice for transaction simulations

* Fix duplicate registration of feemarket GenesisState and Params (PR review)

https://github.com/tharsis/ethermint/pull/1104#issuecomment-1141893712

proto/ethermint/feemarket/v1/feemarket.proto

@@ -25,4 +25,9 @@ message Params {
     (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Int",
     (gogoproto.nullable) = false
   ];
-}
+  // min_gas_price defines the minimum gas price value for cosmos and eth transactions
+  string min_gas_price = 7 [
+    (gogoproto.customtype) = "github.com/cosmos/cosmos-sdk/types.Dec",
+    (gogoproto.nullable) = false
+  ];
+}