MINOR: Adds possibility to use custom FilenameCreator

[gotpist1]

Problem

When writing to S3 there is no possibility to decide on the actual filename that ends up in the bucket. Sometimes, we want to be able to do this to make it more clear on what the file contains and to have more control.
I have described it in this: #631 issue

Solution

I have added an interface called FilenameCreator. This interface can be implemented on a custom class and added as config. If none is provided we fall back to TopicPartitionFilenameCreator that preserves the way that file names where created before this change. So no breaking changes.

Does this solution apply anywhere else?

• [x ] yes
• no

If yes, where?

TopicPartitionWriter now uses a fileName creator, but the change does not affect previous behaviour.
S3SinkConnectorConfig now has a new key: s3.filename.creator.class for specifying your own FilenameCreator

Test Strategy

Changed existing tests to use new FilenameCreator

Testing done:

• [ x] Unit tests
• [ x] Integration tests
• System tests
• [x ] Manual tests

Release Plan

Merging to master

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