CredentialsWrapper doesn't use authCache when building ApplicationDefaultCredentials

[peetya]

Is your feature request related to a problem? Please describe.
I am connecting to different GCP services from GKE using PHP and I am struggling to configure cache for the credentials fetcher in BigtableClient. I defined my cache implementation in BigtableClient through $config['credentialsConfig']['authCache']. As I am not using key files, the CredentialsWrapper will build the ApplicationDefaultCredentials, but instead of using the received parameters, the authCache and the authCacheOptions are hardcoded null values: https://github.com/googleapis/gax-php/blob/main/src/CredentialsWrapper.php#L129-L130

Describe the solution you'd like
I expect these parameters to be used in buildApplicationDefaultCredentials method instead of using null values.

[vam-google]

@bshaffer @noahdietz This looks like something which can be relatively easily fixed, can you please take a quick look?