[Feature] Add a local development kit for contributors

## Motivation

I recently started contributing to Fluss. The build process is clear and the documentation is comprehensive, while most existing guides focus on deployment and usage scenarios. After changing code, running locally built artifacts and validating Flink or lake tiering still requires manually combining several services, configurations, and dependencies.

I am also exploring using coding agents to start environments and validate test cases, which benefits from a stable, clear, and repeatable local interface.

## Solution

I put together a DevKit based on Docker Compose and `just`. It mounts the local `build-target`, manages integration environments through profiles, and provides composable commands for startup and validation.

The DevKit focuses on source development and local validation, alongside the existing quickstarts and deployment guides. A Draft PR provides a working implementation for discussion. Feedback on the direction, tool choice, and maintenance scope would be appreciated.

I am willing to submit a PR.

Review the draft PR's Docker Compose and `just` DevKit, starting with how it mounts `build-target` and manages integration environments through profiles. Run the provided startup and validation commands for Flink and lake-tiering scenarios; done means contributors can repeatedly start local environments and validate locally built artifacts without manually combining services, configurations, and dependencies.

Written by the indexing model from the issue text.