Users cannot provide their own sharding function when using FileIO

Beam uses RandomShardingFunction ([https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L834](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L834)) by default for sharding when using FileIO.write().

RandomShardingFuncction doesn’t work well with Flink Runner. Its assigning same key (hashDestination(destination, destinationCoder))  along with the shard number. ([https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L863](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L863))

This is causing different ShardedKeys going to same task slot. As a result, there is no equal distribution of files written from different task slots.

E.g. 2 files are written by task slot 1, 3 files written by task slot 2, 0 files written by other task slots.

As a result, we are seeing Out of Memory from the pods writing more files.

At [https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L695-L698](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/io/WriteFiles.java#L695-L698)

There is an option to give a different sharding function. But there is no way for the user to mention different sharding function when using FileIO class.

Imported from Jira [BEAM-13667](https://issues.apache.org/jira/browse/BEAM-13667). Original Jira may contain additional context.
Reported by: kathulasandeep.