Codepipeline: implement start_pipeline_execution

[sslogar]

My specific use case guarantees the existence of an active pipeline, but I recognize implementing start_pipeline_execution is a prerequisite to #10190.

[Mide69]

@bblommers I'd like to work on this. Right now CodePipeline's backend only supports pipeline CRUD (create_pipeline, get_pipeline, update_pipeline, list_pipelines, delete_pipeline), there's no execution model at all, not even get_pipeline_execution/list_pipeline_executions.

Plan: add a PipelineExecution store to CodePipelineBackend. Since moto doesn't actually run pipeline actions, start_pipeline_execution will generate a pipelineExecutionId and record the execution as immediately Succeeded, consistent with how moto short-circuits other async AWS flows it doesn't execute for real. I'll also add get_pipeline_execution/list_pipeline_executions so the returned ID is actually retrievable afterward.

Could this be assigned to me?