refactor!: Revamp `apiEndpoint`

[danielbankhead]

With the advent of TPC and PSC, there’s a lot of complexity with apiEndpoints. To improve, we should:

• expose an async Storage#getEndpoint() method
• make the apiEndpoint property private
• remove the useAuthWithCustomEndpoint parameter/property
• remove the internal customEndpoint parameter/property

Additionally, if STORAGE_EMULATOR_HOST is detected and used the PassThrough AuthClient should be used by default (which can be overwritten via the existing authClient parameter) - removing the need for the useAuthWithCustomEndpoint parameter.

This will greatly clean-up the code base as:

• Customers will not be required to manually provide the universeDomain - as we can determine it asynchronously via GoogleAuth#getUniverseDomain. This is a major convenience for TPC customers.
• We can offer a predictable, secure experience by not disabling auth by default when an apiEndpoint has been provided
• Classes will no longer have to distinguish between auth and non-auth contexts - we can simply use auth everywhere uniformly

Related:

• refactor!: Revamp apiEndpoint nodejs-storage#2400
• feat!: Align Support for Storage Emulation nodejs-storage#2092
• feat: PassThrough AuthClient google-auth-library-nodejs#1771

We this change we can remove the following:

nodejs-common/src/util.ts

Lines 140 to 148 in 0b4380a

	/**
	* If true, just return the provided request options. Default: false.
	*/
	customEndpoint?: boolean;
	/**
	* If true, will authenticate when using a custom endpoint. Default: false.
	*/
	useAuthWithCustomEndpoint?: boolean;

nodejs-common/src/util.ts

Lines 766 to 777 in 0b4380a

	const authorizeRequest = async () => {
	if (
	reqConfig.customEndpoint &&
	!reqConfig.useAuthWithCustomEndpoint
	) {
	// Using a custom API override. Do not use `google-auth-library` for
	// authentication. (ex: connecting to a local Datastore server)
	return reqOpts;
	} else {
	return authClient.authorizeRequest(reqOpts);
	}
	};