This is a Bug Report

Description

• What went wrong?
  When following https://serverless.com/framework/docs/providers/aws/events/s3#custom-bucket-configuration you cannot define a custom name with vars for the bucket because the SourceArn of the AWS::Lambda::Permission has to be a String. If the SourceArn is an object it gets _.merge() by the configuration the SLS automatically generates.
• What did you expect should have happened? Overriding the SourceArn with !GetAtt/!Join etc would return a well formed configuration like:

"SourceArn": {
           "Fn::Join": [
                  "-",
                  [
                    "s3test",
                    "dev",
                    "datalake"
                  ]
                ]
        },

Now returns a merge:

"SourceArn": {
          "Fn::Join": [
            "",
            [
              "arn:aws:s3:::",
              {
                "Ref": "AWS::Partition",
                "Fn::Join": [
                  "-",
                  [
                    "s3test",
                    "dev",
                    "datalake"
                  ]
                ]
              },
              ":s3:::datalake"
            ]
          ]
        },

• What was the config you used? https://github.com/ebisbe/s3test/blob/master/serverless.yml
• What stacktrace or error message from your provider did you see?
  The cloudformation stack is invalid with:
  An error occurred: AthenaPartitionerLambdaPermissionDatalakeS3 - Value of property SourceArn must be of type String.

Additional Data

If already made a Fork and I have a possible fix with tests.

• Serverless Framework Version you're using:
• Operating System:
• Stack Trace:
• Provider Error messages: