auth/IdTokenClient should respect eagerRefreshThresholdMillis while checking token expiry

[AyushWalekar]

Environment details

• google-auth-library version: 8.8.0

Issue

Since IdTokenClient extends OAuth2Client, it expected to respect eagerRefreshThresholdMillis value and fetch new idToken within 5 minute of expiry date by default.

Because current check is: (this.credentials.expiry_date || 0) < Date.now()
Ref: https://github.com/googleapis/google-auth-library-nodejs/blob/main/src/auth/idtokenclient.ts#L55

Steps to reproduce

1. Create instance of IdTokenClient with expiry_date
2. It doesn't respect default eagerRefreshThresholdMillis value i.e. 5 minutes before token is about to expire. Rather it fetches new token only after it's expired.

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!