feat/refactor: extend `EthRPC` framework class and use it in `gentest.py`

[danceratopz]

#556 added classes to help calling ETH and Engine API methods on clients, we should extend these classes and refactor gentest.py to use them:

execution-spec-tests/src/cli/gentest.py

Lines 243 to 254 in aa1a571

 def _make_request(self, data) -> requests.Response: 
     error_str = "An error occurred while making remote request: " 
     try: 
         response = requests.post(self.node_url, headers=self.headers, data=json.dumps(data)) 
         if response.status_code >= 200 and response.status_code < 300: 
             return response 
         else: 
             print(error_str + response.text, file=stderr) 
             raise requests.exceptions.HTTPError 
     except requests.exceptions.RequestException as e: 
         print(error_str, e, file=stderr) 
         raise e 

[Eli]

could you please assign this to me?