feat: Add $or logical operator for querying , Student Gradebook CLI

[AjayyXD]

This PR adds a $or logical operator to the filtering engine.

It allows for more flexible queries by enabling users to select rows that match any one of a list of specified conditions. The implementation is backward-compatible and works alongside existing implicit AND filters.

Added a complete, working example application: a command-line "Student Gradebook System."

This demonstrates a practical, real-world use case for CafeDB and showcases its core features, including:

Table creation (db.create_table)

Data insertion (db.insert)

Advanced queries with operators (db.select with $gte)

Data deletion (db.delete)

Database statistics (db.stats)