It's easier for downstream apps like Spark to pick up the new PathOutputCommitter superclass if it is there on 2.10+, even if the S3A committer isn't there.
Adding the interface & binding stuff of
MAPREDUCE-6956 allows for third party committers to be deployed.
I'm not proposing a backport of the
HADOOP-13786 committer: that's Java 8, S3Guard, etc. Too traumatic. All I want here is to allow downstream code to be able to pick up the new interface and so be able to support it and other store committers when available