.NET Compaction - Introduce preconfigured `CompactionStrategy` pattern

Nobody has claimed this yet.

I expect many developers will ask: How do I configured my `CompactionStrategy`? ...which one do I use? ...what thresholds work best?

We can expose a factory that creates a pre-configured `CompactionStrategy` according to a set of options:

```c#
// Setup a chat client for summarization
IChatClient summarizingChatClient = ...;

// Create a compaction strategy based on a menu selection of characteristics
CompactionStrategy tunedStrategy =
CompactionStrategy.Create(
Approach.Balanced,
Size.Compact,
summarizingChatClient);
```

In this example, "size" could be:
- Compact
- Moderate
- Generous

And "approach" could be:
- Aggressive
- Balanced
- Gentle

Perhaps the "size" parameter is informed by the maximum input tokens allowed by a given model, with a default based on a conservative estimate of the most common usage.

Behind the scenes the combination of these two axes map to a certain `CompactionStrategy` configuration. As we improve our configuration, developers using this pattern will automatically realize those improvements.