Allow using other implementations of Format with NumericConfig [LUCENE-6851]

NumericConfig forces me to pass in a java.text.NumberFormat.

Our own internationalisation guidelines say to avoid java.text.NumberFormat and always use com.ibm.icu.text.NumberFormat instead, because the latter does the right thing for Arabic.

I wish NumericConfig would allow me to pass in any Format, so that I didn't have to write the horrible adapter class which I am about to write. :)

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Migrated from [LUCENE-6851](https://issues.apache.org/jira/browse/LUCENE-6851) by Trejkaz