Add CI that runs this repository through a CodePipeline

[HarshCasper]

The repo had no CI. It is the source fixture for LocalStack's CodePipeline tests (localstack-pro test_codepipeline.py pulls main), so this PR adds no sample code and leaves the fixture files untouched.

• New test.sh builds a LocalStack CodePipeline with this repository as its source:
  • Source: CodeStarSourceConnection on $REPOSITORY at $BRANCH
  • Build: CodeBuild with the repo's own buildspec.yml, so the Maven build and the JUnit tests run in CodeBuild
  • Deploy: CloudFormation CREATE_UPDATE on SourceCode::cfn_stack.yaml with a QueueName override
• The pipeline sets pipelineType: V1. Without it, LocalStack created a V2 pipeline, logged Emulation support for V2 type pipelines is not available, and never started an execution. The test now fails after 120s if no execution starts.
• It checks:
  • the pipeline execution succeeds
  • the source action fetched the expected commit (the PR head in CI)
  • the build artifact holds target/messageUtil-1.0.jar, and javap shows MessageUtil with both methods
  • the stack is CREATE_COMPLETE, and its QueueURL output matches the queue
• New workflow, with read-only permissions:
  • pre-pulls the CodeBuild images with retries, because of the CodeBuild image pull race (AWS-2171)
  • passes GITHUB_TOKEN as LOCALSTACK_CODEPIPELINE_GH_TOKEN
  • runs test.sh against the PR branch
  • skips PRs from forks, since the pipeline can only fetch branches in this repository
• README gets a Testing section.

Not covered: the ECS blue/green deployment (appspec.yml, Dockerfile).

Verified by CI.

https://claude.ai/code/session_01DZfDmzUBPqNAu3yFKyfCNM