Use one client session per connector

[enocom]

Currently, we use one client session per instance. Instead, we should use one client session per connector.

cloud-sql-python-connector/google/cloud/sql/connector/instance.py

Lines 184 to 194 in 32c6481

	def _client_session(self) -> aiohttp.ClientSession:
	if self.__client_session is None:
	headers = {
	"x-goog-api-client": self._user_agent_string,
	"User-Agent": self._user_agent_string,
	"Content-Type": "application/json",
	}
	if self._quota_project:
	headers["x-goog-user-project"] = self._quota_project
	self.__client_session = aiohttp.ClientSession(headers=headers)
	return self.__client_session