Add extension point to preprocess data before indexing

[chamilaadhi]

Purpose

Currently there is no mechanism to process registry resource before it is added to the indexing document. With this pr, I have introduced a new interface IndexerPreProcessor.java where we can plug a custom implementation to preprocess the resource before indexing.

With this introduction, In APIM, we prevent registry data migration when when introduce a new property or field to the API

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