Improve Benchmark Tests

**Is your feature request related to a problem? Please describe.**
Brighter does not possess Benchmark tests, unlike Darker. Darker's benchmark tests are very basic, but it does have some. As a result, it is impossible to know if any given change worsens are key flows:

- Processing a Send
- Processing a Publish
- Processing a Publish with multiple subscribers
- Processing a Send that triggers a Publish
- Processing a Send that triggers a Publish to multiple subscribers
- Processing a Post over the InternalBus
- Consuming a message from the InternalBus

Test **both** sync and async paths.

**Describe the solution you'd like**

We should use BenchmarkDotNet to add basic Benchmark tests to Brighter to assure ourselves that we do not slip, or if we do, for valid reasons, at least we sign it off

Start by locating Darker's existing benchmark tests and Brighter's tests or entry points for Send, Publish, the InternalBus, and subscriber handling. Use BenchmarkDotNet and cover both synchronous and asynchronous paths for each listed flow; done means basic benchmarks exist for all seven scenarios and their requested variants.

Written by the indexing model from the issue text.