[FEATURE] Support `CASE` function with Calcite

[LantaoJin]

Is your feature request related to a problem?
Case function implemented in PPL Spark: opensearch-project/opensearch-spark#695
Its syntax should be support in PPL OpenSearch as well.
https://github.com/opensearch-project/opensearch-spark/blob/6832737577119221819c49235c212020c0214fed/ppl-spark-integration/src/main/antlr4/OpenSearchPPLParser.g4#L489-L491