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

Today, some `AuthClient`s have a `scopes` property with of `string | string[]`:

https://github.com/googleapis/google-auth-library-nodejs/blob/4f94ffe474ee41b560813b81e8d621be848d38d0/src/auth/baseexternalclient.ts#L140-L145

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

https://github.com/googleapis/google-auth-library-nodejs/blob/4f94ffe474ee41b560813b81e8d621be848d38d0/src/auth/computeclient.ts#L51-L55

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
- It might be worth looking into storing internally as a `Set`