Add support for partial updates

[hakontro]

This would be more convenient and less error prone for users than having to provide the full DataFrame for updates.

[wjoel]

Would be a great feature.

This likely requires #117 since Spark itself checks that source schema = destination schema on .insertInto, but it would be possible if we had myDf.write.cdpEvents(), myDf.write.cdpDataPoints("tag-123") etc.

[hakontro]

WIP #229

[hakontro]

#229