refactor!: Remove `RefreshOptions` type in `OAuth2Client`

[danielbankhead]

As of https://github.com/googleapis/google-auth-library-nodejs/pull/1663 no clients use this now:

• https://github.com/googleapis/google-auth-library-nodejs/pull/1663

[milinddethe15]

Hi @danielbankhead, I would like to work on this. If I understood the issue, I just have to remove extends RefreshOptions from

export interface OAuth2ClientOptions extends RefreshOptions {
  clientId?: string;
  clientSecret?: string;
  redirectUri?: string;
  credentials?: Credentials;
}

[danielbankhead]

Hey @milinddethe15, good question - after #1663 has been merged the RefreshOptions interface itself can be removed.

[milinddethe15]

Hi, I want to work on this issue. Assign me.

[danielbankhead]

@milinddethe15 done! Take your time as this refactor is a breaking change and we won’t release another major for some time.