ci: add dedicated RAT license check workflow for all PRs

[andygrove]

Summary

• Add a new lightweight CI workflow (pr_rat_check.yml) that runs the Apache RAT license header check on all PRs, including docs-only changes

Problem

The existing build workflows (pr_build_linux, spark_sql_test, etc.) use paths-ignore to skip docs-only changes (matching docs/**, **.md, etc.). This also skips the RAT license check, which allowed files without Apache license headers to be merged via docs-only PRs (e.g., #3651 introduced SVG files without headers, breaking CI for unrelated PRs like #3661).

Solution

A dedicated workflow that:

• Has no path filters — runs on every PR and push to main
• Only runs ./mvnw -B -N apache-rat:check (the -N flag skips child module builds)
• Uses a minimal setup (just Java 11, no Rust/Spark needed)
• Completes in seconds since it only checks license headers

Test plan

• The workflow itself will be validated by CI on this PR

[mbutrovich]

This might be a good opportunity to start using ubuntu-slim. For context, apache/iceberg-rust#2187

ubuntu-slim is a new cost-efficient runner to fit lightweight jobs. We can use this to save ASF infra usage (if possible).

By lightweight I mean jobs that run for less than a minute and are not easily affected by other concurrently running jobs on the same host.

This topic was raised in the Apache ORC project and later adopted by iceberg-cpp. We believe it could also benefit other projects under the Apache Iceberg umbrella.

Refs:

• https://github.blog/changelog/2025-10-28-1-vcpu-linux-runner-now-available-in-github-actions-in-public-preview/
• https://github.com/actions/runner-images/blob/main/images/ubuntu-slim/ubuntu-slim-Readme.md

[mbutrovich]

I opened an issue to track broader ubuntu-slim use. #3665

[mbutrovich]

Thanks @andygrove!