Add compression support to the StringFormat

[omuravskiy]

This change enables the use of CompressionCodecs installed in Hadoop with the StringFormat of HDFS Connector. It introduces a new config parameter - format.string.compression, with a default value of "none", so by default it behaves in the same way as before.

[ghost]

@confluentinc It looks like @omuravskiy just signed our Contributor License Agreement. :+1:

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