This pull request significantly enhances the expression system in flowfile_frame/expr.py by introducing a new internal representation (ff_repr) for expressions, enabling conversion to a "flowfile function" format. It adds mapping logic and propagation for this representation across binary operators, string and datetime methods, casting, and more. Additionally, new string trimming methods are introduced, and support for abs and round operations is added.

Flowfile function representation improvements:

• Added ff_repr attribute to Expr and Column classes, which tracks a flowfile function representation of expressions, and ensures it is propagated through all relevant methods and operations. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

• Introduced mapping dictionaries (FF_OPERATOR_MAP, CAST_FF_MAP, STRING_METHOD_FF_MAP, DT_METHOD_FF_MAP) and helper _get_ff_repr to convert Python/Polars expressions and values into flowfile function syntax.

Operator and method support:

• Implemented flowfile function representation for all binary operators, including both left- and right-side (reverse) operations, as well as for casting, string, and datetime methods. [1] [2] [3] [4] [5]

• Added new methods to Expr for string trimming (strip_chars_start, strip_chars_end), and numeric operations (abs, round), each with their corresponding flowfile function representation. [1] [2]

Utility and consistency improvements:

• Ensured that methods like alias, fill_null, is_null, and is_not_null correctly propagate or compute the ff_repr attribute. [1] [2] [3] [4]

• Updated literal creation (lit) to include flowfile function representation for literal values.