(core) custom codebuild stages

[malachi-constant]

Is your idea related to a problem? Please describe.
Enable users to add their custom CodeBuild stage to the CICD pipeline

Describe the solution you'd like

Something like:

CICDPipelineStack(
        app,
        id="DDKCICDPipelineStack",
        environment_id="dev",
        pipeline_name="ddk-cicd-pipeline",
    )
    .add_source_action(repository_name="ddk-repository")
    .add_synth_action()
    .build()
    .add_stage("dev", ApplicationStage(app, "dev"))
    .add_custom_stage(stage_name="my-custom-stage", commands=["whoami"])

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