feat: service account is able to use a private token endpoint (#784)

In  [Private Service Connect](https://cloud.google.com/vpc/docs/private-service-connect), users can use an endpoint which is private to their VPC network. The request is eventually routed to the oauth2.googleapis.com/token so the "aud" in the assertion still should be oauth2.googleapis.com/token. 

After this change, service account can send requests to the private endpoint (if configured) and still use the oauth2.googleapis.com/token in the assertion. 
diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index dd36589..8f18f26 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -80,6 +80,7 @@
 from google.oauth2 import _client
 
 _DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in seconds
+_GOOGLE_OAUTH2_TOKEN_ENDPOINT = "https://oauth2.googleapis.com/token"
 
 
 class Credentials(
@@ -382,7 +383,7 @@
             # The issuer must be the service account email.
             "iss": self._service_account_email,
             # The audience must be the auth token endpoint's URI
-            "aud": self._token_uri,
+            "aud": _GOOGLE_OAUTH2_TOKEN_ENDPOINT,
             "scope": _helpers.scopes_to_string(self._scopes or ()),
         }
 
@@ -643,7 +644,7 @@
             # The issuer must be the service account email.
             "iss": self.service_account_email,
             # The audience must be the auth token endpoint's URI
-            "aud": self._token_uri,
+            "aud": _GOOGLE_OAUTH2_TOKEN_ENDPOINT,
             # The target audience specifies which service the ID token is
             # intended for.
             "target_audience": self._target_audience,