Refactoring Flint metadata and build index API

[dai-chen]

Code needs refactoring after materialized view added finally:

• Finalize FlintMetadata and refactor serde code:
  • common code moved into flint-core and other left in flint-spark-integration
  • update Flint user reference doc accordingly
• Refactor FlintSpark.createIndex which always uses foreach sink
  • MV build DataFrame doesn't use foreach sink
  • check if covering index can still foreach sink or not

[dai-chen]

Another item: see if previous multi-line JSON string is more readable than XContent use in FlintMetadata. May track separately.