Support `AuthClient` for `auth`

[danielbankhead]

Currently the standard auth parameter uses an explicit list of clients that extend AuthClient:

auth?:
      | string
      | OAuth2Client
      | JWT
      | Compute
      | UserRefreshClient
      | BaseExternalAccountClient
      | GoogleAuth;

Src: https://github.com/googleapis/google-api-nodejs-client/blob/main/src/apis/abusiveexperiencereport/v1.ts#L48-L55

In order to support new clients, such as DownscopedClient, auth can be simplified to:

auth?: string | AuthClient | GoogleAuth;

Related:

• TypeScript issue: Type 'JSONClient | Compute' is not assignable to type 'JSONClient' #779
• feat: support GoogleAuth parameter google-api-nodejs-client#2210

[danielbankhead]

Here's a permalink:

https://github.com/googleapis/google-api-nodejs-client/blob/9a80a59df4a882bfca09a19f30556ef05772488a/src/apis/abusiveexperiencereport/v1.ts#L47-L54