Add Optimism mainnet and kovan networks

Added OPTIMISM mainnet and kovan tesnet id's and URLs
ethereum-optimism/optimism#983


Also, @smartcontracts from OP says hi 👋 ❤

packages/hardhat-etherscan/src/network/prober.ts

@@ -30,6 +30,9 @@ enum NetworkID {
   HECO_TESTNET = 256,
   // Fantom mainnet
   OPERA = 250,
+  //Optimistim
+  OPTIMISM = 10,
+  OPTIMISM_KOVAN = 69,
 }
 
 const networkIDtoEndpoints: NetworkMap = {
@@ -73,6 +76,14 @@ const networkIDtoEndpoints: NetworkMap = {
     apiURL: "https://api.ftmscan.com/api",
     browserURL: "https://ftmscan.com",
   },
+  [NetworkID.OPTIMISM]: {
+    apiURL: "https://api-optimistic.etherscan.io",
+    browserURL: "https://optimistic.etherscan.io/",
+  },
+  [NetworkID.OPTIMISM_KOVAN]: {
+    apiURL: "https://api-kovan-optimistic.etherscan.io",
+    browserURL: "https://kovan-optimistic.etherscan.io/",
+  },
 };
 
 export async function getEtherscanEndpoints(