window.ethereum.selectedAddress is not checksummed

**Describe the bug**
window.ethereum.selectedAddress has an output in lowercase, while the displayed wallet address in the UI (including when you copy it to a clipboard) is in mixed case. While this is fine for web3 apps that are not case sensitive, it is not the case for identity applications where the wallet address acts as a unique username. This leads to failure in login because the username doesn't match, e.g. implementation with Eauth wallet login (Eauth is an OAuth2 authentication server: https://github.com/pelith/node-eauth-server ).

**Steps to reproduce (REQUIRED)**
Steps to reproduce the behavior, libraries used with version number, and/or any setup information to easily reproduce:

1. Connect the MetaMask
2. On browser console, check the output of window.ethereum.selectedAddress; and notice it is in small case
3. Click the MetaMask extension badge and copy the wallet address to clipboard; notice the copied address is in mixed case

**Expected behavior**
Both output should match.

**Browser details (please complete the following information):**
- OS: Any
- Hardware Wallet: Any
- Browser: Any
- MetaMask Version: 10.2.2

Start at the window.ethereum.selectedAddress provider entry point and trace how the exposed address is produced, comparing it with the extension's copied wallet address. Reproduce the mismatch using the listed browser-console steps, then verify that both outputs match and run the relevant provider tests if present.

Written by the indexing model from the issue text.