feat(cu): add util for getting chain ID (#2509)

Adds a one-liner for getting chain ID given an ethers.Provider input.
Going to follow this PR up with a good first issue to use this util
wherever we're trying to get the chain ID.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/warm-points-enjoy.md

+---
+'@eth-optimism/core-utils': patch
+---
+
+Adds a one-liner for getting chain ID from provider

packages/core-utils/src/external/ethers/index.ts

@@ -3,3 +3,4 @@
  */
 
 export * from './fallback-provider'
+export * from './network'

packages/core-utils/src/external/ethers/network.ts

+import { Provider } from '@ethersproject/abstract-provider'
+
+export const getChainId = async (provider: Provider): Promise<number> => {
+  const network = await provider.getNetwork()
+  return network.chainId
+}