Add ExcelHLoader for loading Excel files with header detection

[harikrishnancj]

Description

This PR adds ExcelHLoader, a document loader for Excel files (.xlsx, .xls) with intelligent row-by-row data extraction.

Features

• Loads Excel files and converts each row to a LangChain Document
• Handles different data types (numbers, dates, missing values)
• Supports column selection
• Metadata includes row index

Implementation

• Created excel_h_loader.py with the ExcelHLoader class
• Added proper imports to __init__.py
• Added to __all__ exports

Testing

• Tested with sample Excel files containing various data types
• Verified proper Document creation and metadata