Is there a Lambda trigger that will push/copy files from S3 bucket to the SFTP server?
You have to use the following algorithm: 

Log in to AWS console,
Lambda service,
Create a new lambda function (choose a proper runtime);

Then you have to set a trigger (a left pane). If you want to trigger your lambda by s3 bucket then click on s3. 
Then you have to configure your trigger (choose your bucket and trigger action). 
After you complete all of this steps it's a time to write a handler file.
Do not forget that every single trigger has to be in the same region as lambda.
I am looking for a way to set up a Lambda trigger, where if any files are uploaded into the S3 bucket, the trigger will push/copy the file(s) to the SFTP server.