feat!: Normalize #scopes args and properties to string[]

[danielbankhead]

Today, some AuthClients have a scopes property with of string | string[]:

google-auth-library-nodejs/src/auth/baseexternalclient.ts

Lines 140 to 145 in 4f94ffe

	/**
	* OAuth scopes for the GCP access token to use. When not provided,
	* the default https://www.googleapis.com/auth/cloud-platform is
	* used.
	*/
	public scopes?: string | string[];

And we have to normalize in other places like this with code like this:

google-auth-library-nodejs/src/auth/computeclient.ts

Lines 51 to 55 in 4f94ffe

	this.scopes = Array.isArray(options.scopes)
	? options.scopes
	: options.scopes
	? [options.scopes]
	: [];

This is pretty inconsistency throughout the library. Additionally, this change will clarify to customers that may be confused as to if a string-typed scopes property will be parsed into multiple scopes are not.

Implementation Notes:

• Ensure samples are providing the recommended string[] as parameters