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

[murluki]

could you please assign this to me?