Refactor Flint index metadata

[dai-chen]

Description

1. Refactor FlintMetadata with specific strong typed field instead of previous generic content string. (recreate the class in Scala for readability)
2. Make use of existing XContent lib without adding extra JSON dependency to flint-core for now.
3. Remove all JSON parsing and building logic in flint-spark-integration module.

Questions

1. In ca44449, I missed the changes in FlintJob however all CI passed. sbt integtest/test seems passed even local sbt test failed.
2. Meanwhile the use of FlintMetadata in FlintJob may need more UT/IT to cover.

cc: @kaituo

Issues Resolved

#24

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