feat: Provide Mechanism to Change AuthClient Scopes Uniformly

[danielbankhead]

Today, we don't have a clean way to change AuthClient scopes uniformly between instances, and different AuthClients offer varying functionality like this:

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

Lines 100 to 115 in 6c0a6bd

	/**
	* Creates a copy of the credential with the specified scopes.
	* @param scopes List of requested scopes or a single scope.
	* @return The cloned instance.
	*/
	createScoped(scopes?: string | string[]) {
	return new JWT({
	email: this.email,
	keyFile: this.keyFile,
	key: this.key,
	keyId: this.keyId,
	scopes,
	subject: this.subject,
	additionalClaims: this.additionalClaims,
	});
	}

We should offer a uniform API for changing scopes for all AuthClients. Additionally, once this is implemented GoogleAuth#scopes should update its cachedCredentials's scopes (perhaps via a simple getter/setter hook on GoogleAuth itself).

Dependencies:

• feat: Unify Base AuthClient Options #1663
• feat!: Normalize #scopes args and properties to string[] #1676