Add Etherscan configuration for new networks (#240)

## Description

Add the necessary configuration lines for allowing contract verification
on the Block explorers of the networks introduced in #239.
BNB was already verified. Optimism and Polygon just worked out of the
box. Avalanche defaults to [Snowtrace](https://snowtrace.io/), so I had
to include a custom configuration for that network in order to use
[SnowScan](https://snowscan.xyz/) and used:

```sh
npx hardhat verify --network avalanche 0x9E7Ae8Bdba9AA346739792d219a808884996Db67
npx hardhat verify --network avalanche 0x9008D19f58AAbD9eD0D60971565AA8510560ab41 0x2c4c28DDBdAc9C5E7055b4C863b72eA0149D8aFE 0xBA12222222228d8Ba445958a75a0704d566BF2C8
```

I also enabled Sourcify because why not?

## Test Plan

Check out the contract addresses on the block explorer.

---------

Co-authored-by: Mateo <mateo@cow.fi>

Author: fedgiac

hardhat.config.ts

@@ -187,11 +187,17 @@ export default {
     currency: "USD",
     gasPrice: 21,
   },
+  sourcify: {
+    enabled: true,
+  },
   etherscan: {
     apiKey: {
       sepolia: ETHERSCAN_API_KEY,
       arbitrumOne: ETHERSCAN_API_KEY,
       base: ETHERSCAN_API_KEY,
+      optimisticEthereum: ETHERSCAN_API_KEY,
+      polygon: ETHERSCAN_API_KEY,
+      avalanche: ETHERSCAN_API_KEY,
     },
     customChains: [
       {
@@ -202,6 +208,14 @@ export default {
           browserURL: "https://basescan.org",
         },
       },
+      {
+        network: "avalanche",
+        chainId: 43114,
+        urls: {
+          apiURL: "https://api.snowscan.xyz/api",
+          browserURL: "https://snowscan.xyz",
+        },
+      },
     ],
   },
 };