added check for fee recipient per validator and added unit tests (#8454)

Addresses #5403


  - Added `check_fee_recipient()` method to validate individual validators
- Added `check_all_fee_recipients()` to validate all validators on startup
- Validator client now fails to start if any enabled validator lacks a fee recipient and no global flag is used.
- Added Clear error messages to guide users on how to fix the issue
- Added unit tests


Co-Authored-By: AbolareRoheemah <roheemahabo@gmail.com>

validator_client/src/lib.rs

@@ -187,6 +187,9 @@ impl<E: EthSpec> ProductionValidatorClient<E> {
             info!(new_validators, "Completed validator discovery");
         }
 
+        // Check for all validators' fee recipient
+        validator_defs.check_all_fee_recipients(config.validator_store.fee_recipient)?;
+
         let validators = InitializedValidators::from_definitions(
             validator_defs,
             config.validator_dir.clone(),