feat!: Normalize GoogleAuth<T> to AuthClient

[danielbankhead]

We should normalize GoogleAuth<T extends AuthClient = JSONClient> to GoogleAuth<T extends AuthClient = AuthClient> (and GoogleAuthOptions as well).

Additionally, we should:

• change clientOptions from:
  

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

  Lines 106 to 114 in 4f94ffe

  	/**
  	* Options object passed to the constructor of the client
  	*/
  	clientOptions?:
  	| JWTOptions
  	| OAuth2ClientOptions
  	| UserRefreshClientOptions
  	| ImpersonatedOptions;

  
  to:

  /**
   * Options object passed to the constructor of the client
   */
  clientOptions?: ConstructorParameters<typeof T>[0];

  In both GoogleAuth and GoogleAuthOptions to accurately match the expected client options.

• change cachedCredential from:
  

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

  Lines 160 to 161 in f950465

  	cachedCredential: JSONClient | Impersonated | Compute | T | null = null;

  
  to:

  cachedCredential: T | null = null; 

  in order to normalize the expected AuthClient

Dependencies:

• feat: Backwards-Compatible Auth #1624
• feat: Unify Base AuthClient Options #1663
• refactor!: Remove additionalOptions from AuthClients #1672

Implementation Notes:

• Look into overload narrowing (which is a pain at the time of writing), which is required for accurate ConstructorParameters results:
  • ConstructorParameters<T> cannot get correct types of overloaded constructor microsoft/TypeScript#37079
  • https://stackoverflow.com/questions/59745527/is-it-possible-to-narrow-the-types-of-overloaded-parameters-without-exhaustively
  • Note: We may just have to reorganize overloads in each AuthClient to the desired overload signature as the first overload ('desired' = whichever one exposes the extended AuthClientOptions as the first option).