Draft: chore: Add an end-to-end batching benchmark

[orf]

This is a bit of a weird PR, sorry. I have been playing a lot with the internals of Vector recently for a project of mine to integrate Vector as a Python library. Vector is a really nicely structured project, and after a few wrong turns I was able to integrate it in-proces with Python as a library.

To explore how everything works (especially the service abstractions) I built the following dummy source and sink, and I figured I'd try and contribute it as a end-to-end Criterion benchmark for a couple of reasons:

1. I found it quite hard to find completely minimal/bare-bones non-test sources and sinks, so perhaps this could form an example for contributors?
2. An end-to-end benchmark involving producing and consuming might be an useful thing to include?
   • I was curious about potential overhead incurred with having to repeatedly allocate Vec's to call SourceSender::send_batch and I had no idea where to start profiling this.

Feel free to close this, but I figured it might be useful to someone in the future and I didn't want it to go to waste.

[bits-bot]

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