Using the default http client for getting public keys

[hiranya911]

We have been using the same authorized HTTP client to retrieve public keys when verifying ID tokens. But since recently, the back-end server does not allow it. When called with credentials, it now returns the following error:

{
  "error": {
    "code": 400,
    "message": "Request contains an invalid argument.",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.DebugInfo",
        "detail": "No scopes defined for OAuth: serviceaccountcert.googleapis.com/google.iam.credentials.v1.CertificateService.ExternalListPublicCertificates"
      }
    ]
  }
}

This caused several of our integration tests fail.

Fixing the issue by using the default HTTP client to retrieve public keys.