feat: use self-signed jwt for service account (#665)

diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index c4898a2..ed91011 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -126,6 +126,7 @@
         service_account_email,
         token_uri,
         scopes=None,
+        default_scopes=None,
         subject=None,
         project_id=None,
         quota_project_id=None,
@@ -135,8 +136,10 @@
         Args:
             signer (google.auth.crypt.Signer): The signer used to sign JWTs.
             service_account_email (str): The service account's email.
-            scopes (Sequence[str]): Scopes to request during the authorization
-                grant.
+            scopes (Sequence[str]): User-defined scopes to request during the
+                authorization grant.
+            default_scopes (Sequence[str]): Default scopes passed by a
+                Google client library. Use 'scopes' for user-defined scopes.
             token_uri (str): The OAuth 2.0 Token URI.
             subject (str): For domain-wide delegation, the email address of the
                 user to for which to request delegated access.
@@ -155,6 +158,7 @@
         super(Credentials, self).__init__()
 
         self._scopes = scopes
+        self._default_scopes = default_scopes
         self._signer = signer
         self._service_account_email = service_account_email
         self._subject = subject
@@ -162,6 +166,8 @@
         self._quota_project_id = quota_project_id
         self._token_uri = token_uri
 
+        self._jwt_credentials = None
+
         if additional_claims is not None:
             self._additional_claims = additional_claims
         else:
@@ -249,11 +255,12 @@
         return True if not self._scopes else False
 
     @_helpers.copy_docstring(credentials.Scoped)
-    def with_scopes(self, scopes):
+    def with_scopes(self, scopes, default_scopes=None):
         return self.__class__(
             self._signer,
             service_account_email=self._service_account_email,
             scopes=scopes,
+            default_scopes=default_scopes,
             token_uri=self._token_uri,
             subject=self._subject,
             project_id=self._project_id,
@@ -275,6 +282,7 @@
             self._signer,
             service_account_email=self._service_account_email,
             scopes=self._scopes,
+            default_scopes=self._default_scopes,
             token_uri=self._token_uri,
             subject=subject,
             project_id=self._project_id,
@@ -301,6 +309,7 @@
             self._signer,
             service_account_email=self._service_account_email,
             scopes=self._scopes,
+            default_scopes=self._default_scopes,
             token_uri=self._token_uri,
             subject=self._subject,
             project_id=self._project_id,
@@ -314,6 +323,7 @@
         return self.__class__(
             self._signer,
             service_account_email=self._service_account_email,
+            default_scopes=self._default_scopes,
             scopes=self._scopes,
             token_uri=self._token_uri,
             subject=self._subject,
@@ -357,10 +367,30 @@
 
     @_helpers.copy_docstring(credentials.Credentials)
     def refresh(self, request):
-        assertion = self._make_authorization_grant_assertion()
-        access_token, expiry, _ = _client.jwt_grant(request, self._token_uri, assertion)
-        self.token = access_token
-        self.expiry = expiry
+        if self._jwt_credentials is not None:
+            self._jwt_credentials.refresh(request)
+            self.token = self._jwt_credentials.token
+            self.expiry = self._jwt_credentials.expiry
+        else:
+            assertion = self._make_authorization_grant_assertion()
+            access_token, expiry, _ = _client.jwt_grant(
+                request, self._token_uri, assertion
+            )
+            self.token = access_token
+            self.expiry = expiry
+
+    def _create_self_signed_jwt(self, audience):
+        """Create a self-signed JWT from the credentials if requirements are met.
+
+        Args:
+            audience (str): The service URL. ``https://[API_ENDPOINT]/``
+        """
+        # https://google.aip.dev/auth/4111
+        # If the user has not defined scopes, create a self-signed jwt
+        if not self.scopes:
+            self._jwt_credentials = jwt.Credentials.from_signing_credentials(
+                self, audience
+            )
 
     @_helpers.copy_docstring(credentials.Signing)
     def sign_bytes(self, message):