(unified) GC: Improve resiliance for unexpected files under _lakefs

[yonipeleg33]

Currently, if users upload files to the _lakefs prefix, the GC will fail to run with an ugly error:

Caused by: org.apache.spark.SparkException: Job aborted due to stage failure: Task 88 in stage 7.0 failed 4 times, most recent failure: Lost task 88.3 in stage 7.0 (TID 42180) ([2a05:d018:179b:7f01:d8e3:50aa:4d29:3899] executor 121): io.treeverse.jpebble.BadFileFormatException: Bad magic 37 66 30 31 22 0a 7d 0a: wrong bytes

For context, when we added the dummy file under _lakefs we had to explicitly ignore it in the GC.
Back then, we had the idea of whitelisting only metadata files, but were against it.

We need to revisit this decision, or come up with other ideas to improve the GC's resistance for such cases.