Add a RawTool : AITool type

[stephentoub]

For cases where a developer wants to use a specific tool for which there's no abstract representation, we could add a class:

public sealed class RawTool : AITool
{
    public object? RawRepresentation { get; set; }
    ...
}

Then if someone has a MyCoolAnthropicTool that's known to the underlying implementation, the developer can do:

MyCoolAnthropicTool tool = ...;
ChatOptions options = new() { Tools = [.. new RawTool(tool)] };

Leaf clients would special-case RawTool to fish out the RawRepresentation and use it directly in their processing.

We could do the same thing for other things, like response format.

[stephentoub]

This may not make sense anymore. If you know who you're talking to, you can also now use ChatOptions.RawRepresentationFunc to supply a pre-configured options instance, with the relevant tool already set on it.