feat: use self-signed JWTs with ServiceAccountCredentials in ConnectorCredentialsProvider (#433)

Author: whowes

src/main/java/com/google/pubsub/kafka/common/ConnectorCredentialsProvider.java

@@ -71,6 +71,7 @@ public static ConnectorCredentialsProvider getServiceAccountFromFile(String cred
     return new ConnectorCredentialsProvider(
         () ->
             ServiceAccountCredentials.fromStream(new FileInputStream(credentialsSAPath))
+                .createWithUseJwtAccessWithScope(true)
                 .createScoped(GCP_SCOPE));
   }
 
@@ -79,6 +80,7 @@ public static ConnectorCredentialsProvider getServiceAccountFromJson(String cred
         () ->
             ServiceAccountCredentials.fromStream(
                     new ByteArrayInputStream(credentialsSAJson.getBytes()))
+                .createWithUseJwtAccessWithScope(true)
                 .createScoped(GCP_SCOPE));
   }