feat: introduce database level ops for cpp bindings

[luoyuxia]

Purpose

Linked issue: close #274

Brief change log

Tests

API and Format

Documentation

[Copilot]

Pull request overview

This PR adds database-level administrative operations to the C++ bindings, aligning the C++ Admin API with the underlying Rust FlussAdmin capabilities requested in issue #274.

Changes:

• Extended the Rust CXX bridge with database structs/results and added Admin FFI methods (create/drop/list DBs, existence checks, info lookup, list tables, drop partition).
• Added Rust/C++ conversion helpers for DatabaseDescriptor and DatabaseInfo.
• Exposed new C++ Admin APIs in the public header and provided a new admin_example.cpp plus build targets (CMake + Bazel).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
bindings/cpp/src/types.rs	Adds conversions between FFI and core database metadata types.
bindings/cpp/src/lib.rs	Extends the CXX bridge and implements new Admin database/partition operations.
bindings/cpp/src/ffi_converter.hpp	Adds C++ ↔ FFI converters for database descriptor/info.
bindings/cpp/src/admin.cpp	Adds C++ Admin wrapper methods for new database ops and drop_partition.
bindings/cpp/include/fluss.hpp	Exposes DatabaseDescriptor, DatabaseInfo, and new Admin methods in the public C++ API.
bindings/cpp/examples/admin_example.cpp	New example demonstrating database + table admin operations.
bindings/cpp/CMakeLists.txt	Adds a CMake target for the new admin example binary.
bindings/cpp/BUILD.bazel	Adds a Bazel target for the new admin example binary.

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[luoyuxia]

@fresh-borzoni @zhaohaidao Could you please help review?

[fresh-borzoni]

@luoyuxia TY for the PR.
LGTM 👍