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

validator_client/src/http_api/tests.rs

@@ -25,7 +25,7 @@ use std::sync::Arc;
 use tempfile::{tempdir, TempDir};
 use tokio::sync::oneshot;
 
-const PASSWORD_BYTES: &[u8] = &[42, 13, 37];
+const PASSWORD_BYTES: &[u8] = &[42, 50, 37];
 
 type E = MainnetEthSpec;