ci: add Makefile and GitHub Actions workflow for PR checks

[wombatu-kun]

Summary

Adds the canonical local build entry point (Makefile) and the main CI
workflow (.github/workflows/flink.yml).

After merge, every PR against main is gated by two jobs:

• lint — mvn spotless:check checkstyle:check
• build-and-test — mvn install -DskipTests followed by mvn test

Both run on Ubuntu 24.04 with Temurin 8 (matching
maven.compiler.target=1.8 and Flink 1.16.1 native Java) and use the
actions/setup-java Maven cache. Concurrency is configured to cancel
in-progress runs when new commits land on a PR.

Stacking

This PR is stacked on top of build/code-style-setup (Spotless,
Checkstyle, Enforcer plugin configuration) and chore/apply-initial-formatting
(initial spotless:apply pass over the existing sources). The PR diff
currently includes their commits as well; once both upstream PRs merge
and this branch is rebased onto main, only the two new files remain.

Forward compatibility

The Makefile exposes install-all, test-all, build-all aliases
that currently forward to their single-module counterparts. These are
reserved for a future multi-module iteration pattern over
FLINK_VERSION, so .github/workflows/flink.yml will not need to be
rewritten when that lands.