Render pipeline dependency graphs asynchronously

[karkhaz]

Currently, Litani forks out to dot in the main thread. This blocks report generation (and other aspects of the Litani run) from happening until all pipeline dependency graphs are completed (i.e. one invocation of dot for every pipeline).

What should happen instead is that the PipelineDepgraphRenderer render method should be a coroutine, which uses asyncio or threading to spawn dot. This will happen in its own process, freeing the main litani run-build thread. The coroutine should update the "dependencies_url" SVG only when the SVG gets written. The Jinja template should check, for each pipeline, whether the "dependencies_url" key exists and is set; if not, it will simply not add the dependency graph to the pipeline page.