[Umbrella][FIP-49] Implement the Fluss Gateway REST API

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

FIP-49 has been accepted to add a stateless, multi-cluster REST Gateway for Apache Fluss. The Gateway exposes metadata discovery, DDL, writes, primary-key lookups, and prefix lookups to HTTP clients while preserving Fluss authentication and authorization semantics.

- FIP: https://cwiki.apache.org/confluence/spaces/FLUSS/pages/444334561/FIP-49+Fluss+Gateway+REST+API
- Discussion: https://lists.apache.org/thread/s5yjysvoj8o6nxdj49m99hg8yn51tgjw
- Vote: https://lists.apache.org/thread/t5jkcvo8b07oxjywbckotbsxl8fhcd76
- Vote result: https://lists.apache.org/thread/f4217jbyy1dv139ml9t5ybwkhsymy2xm

A proof of concept implements most of the proposal, but it is too large to review as one pull request. This umbrella tracks capability-sized issues and review units that can be tested and merged independently.

### Sub-tasks

The work is split along three parallel lines so each unit can be reviewed and merged independently: capability units run against the `GatewayBackend` port with the testing backend and do not wait for native connectivity; native-connectivity units are config-gated and land in parallel; delivery items trail the rest.

**Foundation**

- [x] #4002
- [x] #3958
- [x] #3969

**Capability line**

- [x] #4067
- [x] #4082
- [x] #4083
- [x] #4084

**Delivery**

- [x] #4085
- [x] #4160

**Future Work**
- [ ] #4081
- [ ] [FIP-49][gateway] Add batched primary-key and prefix lookups with admission limits
- [ ] [FIP-49][gateway] Wrap up: complete the Prometheus metric families with a checklist regression, add user documentation, validate the OpenAPI specification, and execute the dockerized end-to-end quickstart
- [ ] [FIP-49][gateway] And Propagate HTTP principals to Fluss with bounded act-as connection pools

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start with the FIP-49 proposal and the unchecked Future Work items; this issue is an umbrella, not a self-contained implementation task. Use the GatewayBackend port and testing backend for capability work, and review the listed Prometheus, documentation, OpenAPI, and dockerized end-to-end deliverables to define what remains and what completion requires.

Written by the indexing model from the issue text.