Add comparators for system column types

[giant0791]

Add specialized comparators to enable equality checks for system type engines.
Examples:

# test for identity equality
expr = students.sys.database_id == '...'

# visibility test: explicit test for non deleted rows
stmt = select(students)
stmt.where(students.sys.is_deleted.is_(False) & students.c.grade > 2.0)

# temporal data
stmt = select(students)
stmt.where(
    students.sys.created_at.after("2024-12-31") 
    & students.sys.updated_at.before("2025-12-31")
)