Add support for index compression

Add compress attribute to createIndex change allowing creation of compressed index on Oracle, like

( n is optional and specifies number of index prefix columns to compress ; see https://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_5011.htm#i2127535 "key_compression" for details)

The compression should be silently ignored on databases that do not support index compression.

Best, if API allowed future support for other databases that might have different compression settings. From my point of view, it's sufficient if the attribute is actually called oracleCompression.