Improve caching of Instance configs

[jackwotherspoon]

Currently we cache instance connection info by instanceConnectionName and error out if the iam authn flag switches between consecutive connect calls. We should update the hash key (instanceConnectionName+iam authn) for how we cache our configs so that we can handle support for flag switches without needing to error.

cloud-sql-nodejs-connector/src/connector.ts

Lines 76 to 79 in 00a6eae

	// in case an instance to that connection name has already
	// been setup there's no need to set it up again
	if (this.has(instanceConnectionName)) {
	const instance = this.get(instanceConnectionName);