[TASK-251] Support Decimals in CPP

[fresh-borzoni]

Summary

closes #251

Add decimal type support to C++ bindings

Adds DECIMAL(p, s) support to the C++ bindings, matching the Python bindings' schema-driven pattern.

Write path

• User calls row.SetDecimal(idx, "123.45") — just a string, no precision/scale needed
• String crosses FFI as-is (DATUM_TYPE_DECIMAL_STRING)
• Rust parses via BigDecimal::from_str, looks up (p, s) from the schema, validates, and creates the Decimal

Read path

• Arrow Decimal128 values come back as DecimalI64 (compact, p≤18) or DecimalI128 (non-compact)
• Datum::DecimalToString() formats them for display

[fresh-borzoni]

@luoyuxia @zhaohaidao
Appreciate your review here. PTAL 🙏

[Copilot]

Pull request overview

Adds DECIMAL(p, s) support to the C++ bindings, aligning with the schema-driven approach used by other bindings.

Changes:

• Extend the C++/Rust FFI schema (FfiColumn) to carry decimal precision/scale and map it into core metadata.
• Add write-path decimal support from C++ via GenericRow::SetDecimal(...) (string over FFI) and Rust-side parsing/validation.
• Add read-path decimal support by converting Arrow Decimal128 columns into DatumType::DecimalI64 / DecimalI128 and formatting via Datum::DecimalToString().

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
crates/fluss/Cargo.toml	Switch bigdecimal to workspace dependency.
Cargo.toml	Add bigdecimal to [workspace.dependencies] for shared use.
bindings/cpp/Cargo.toml	Add bigdecimal dependency for C++ binding Rust crate.
bindings/cpp/src/types.rs	Add decimal schema mapping, decimal write parsing, and decimal read conversion from Arrow.
bindings/cpp/src/lib.rs	Pass schema to row conversion for decimal validation by storing TableInfo in AppendWriter.
bindings/cpp/src/ffi_converter.hpp	Marshal new decimal-related fields across C++ <-> Rust FFI structs.
bindings/cpp/include/fluss.hpp	Add Decimal types/fields and decimal formatting helpers to the public C++ API.
bindings/cpp/examples/example.cpp	Add a C++ example demonstrating decimal schema + write/read.

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[fresh-borzoni]

rebased and addressed comments