Reject invalid utf-8 characters during encryption (#1928)

## Issue Addressed

Closes #1889 

## Proposed Changes

- Error when passwords which use invalid UTF-8 characters during encryption. 
- Add some tests

## Additional Info

I've decided to error when bad characters are used to create/encrypt a keystore but think we should allow them during decryption since either the keystore was created
-  with invalid UTF-8 characters (possibly by another client or someone whose password is random bytes) in which case we'd want them to be able to decrypt their keystore using the right key.
-  without invalid characters then the password checksum would almost certainly fail.

Happy to add them to decryption if we want to make the decryption more trigger happy 😋 , it would only be a one line change and would tell the user which character index is causing the issue.

See https://eips.ethereum.org/EIPS/eip-2335#password-requirements

common/eth2_wallet_manager/src/wallet_manager.rs

@@ -299,7 +299,7 @@ mod tests {
         );
 
         for i in 1..3 {
-            w.next_validator(WALLET_PASSWORD, &[1], &[0])
+            w.next_validator(WALLET_PASSWORD, &[50; 32], &[51; 32])
                 .expect("should create validator");
             assert_eq!(
                 load_wallet_raw(&base_dir, &uuid).nextaccount(),