In this PR, we add the capability for Visbuilder to generate dynamic Vega and Vega-Lite
specifications based on user settings and aggregation configurations.

* developed functions buildVegaSpecViaVega and buildVegaSpecViaVegaLite
that can create either Vega or Vega-Lite specifications depending on the complexity
of the visualization.
* added VegaSpec and VegaLiteSpec interfaces to provide better type checking
* broken down the specification building into smaller, reusable components
(like buildEncoding, buildMark, buildLegend, buildTooltip) to make the code
more maintainable and easier to extend.
* added flattenDataHandler to prepare and transform data for use in Vega visualizations

Issue Resolve
opensearch-project#7067

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

* update file and functions names
* fix type errors
* fix area chart

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

Signed-off-by: Anan Zhuang <ananzh@amazon.com>

Signed-off-by: Anan Zhuang <ananzh@amazon.com>