Speed up immutable json index build time

[wirybeaver]

previously have an internal discussion with the team on translating mutable json index onto the immutable json index to avoid repeatedly parsing and flatten json docs. But It was temporarily blocked due to the fact that flattened doc id of mutable / immutable segments are different and lack of some interfaces. https://github.com/apache/pinot/pull/12744 has adds the mapping of ids in the IndexCreationContext. Thus, I plan to implement the initial idea this month