BKDWriter refactor: Make BKDIndexWriter an interface [LUCENE-9294]

With this change we introduce an interface called BKDIndexWriter. This interface should be just a wrapper of an IndexOutput, therefore whenever we are creating a BKD index we should pass an object that implements this interface instead of an IndexOutput.

This change allows to remove the SimpleTextBKDWriter fork and replace it with a SimpleTextBKDIndexWriter, therefore have just the logic to build a tree in one place.

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Migrated from [LUCENE-9294](https://issues.apache.org/jira/browse/LUCENE-9294) by Ignacio Vera (@iverase)

Start by locating BKDIndexWriter, IndexOutput, and the existing SimpleTextBKDWriter implementation in the Lucene source. Trace where BKD indexes are created and identify the affected callers; done means BKD construction accepts the new interface and the SimpleText fork is replaced by SimpleTextBKDIndexWriter without duplicating tree-building logic.

Written by the indexing model from the issue text.