Add Scigen table2text task with llm_as_judge metric

[csrajmohan]

Add Scigen dataset for table2text task with llm_as_judge metric using llama3 as judge model This includes,

• addition of scigen task card
• addition of a llm_as_judge metric and prompt template for table2text
• addition of a structured data operator (name: ConstructTableFromRowsCols, Purpose: Builds a single table in standard format by mapping column and row fields into one structure)