Change Postings Format [LUCENE-7881]

[asfimport]

I would like to customize the postings format in lucene to hold more information about each token. I am looking into FreqProxPostingArray. Is this the actual data structure that holds the postings? I also looked at SimpleTextFieldsWriter. What other files should I look at in order to see the changes in index?

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Migrated from LUCENE-7881 by Jaewoo Kim (@jaywoo123)