[API Proposal]: ChatToolMode.RequireAnyOf

### Background and motivation

MEAI currently supports 3 mode to control the output of the LLM:
- Auto: return tools or natural language
- RequireAny: only return tools
- RequireSpecific: only return this single tool

Providers also support a "RequireSpecific" but with a list of tools:
- [tool_choice.tools](https://platform.openai.com/docs/api-reference/responses/create#responses-create-tool_choice-allowed-tools-tools) for OpenAI
- [toolConfig.functionCallingConfig.allowedFunctionNames](https://ai.google.dev/api/caching#FunctionCallingConfig) for Google
- Not supported with [Mistral](https://docs.mistral.ai/api/endpoint/chat#operation-chat_completion_v1_chat_completions_post). Should fallback on ANY
- Not supported with [Claude](https://docs.claude.com/en/api/messages#body-tool-choice). Should fallback on ANY

This proposal aims to support that.

### API Proposal

```diff
public class ChatToolMode
{
+ public static RequiredChatToolMode RequireAnyOf(IList functionNames);
}

public sealed class RequiredChatToolMode : ChatToolMode
{
///
/// Gets the name of a specific tool that must be called.
///
///
/// If the value is , any available tool can be selected (but at least one must be).
+ /// If multiple functions are required, this property returns .
///
public string? RequiredFunctionName { get; }
+
+ ///
+ /// Gets the names of specific tools that must be called.
+ ///
+ ///
+ /// If the value is , any available tool can be selected (but at least one must be).
+ ///
+ public IList? RequiredFunctionNames { get; }

public RequiredChatToolMode(string? requiredFunctionName) { }
+ public RequiredChatToolMode(IList requiredFunctionNames) { }
}
```

I think it makes sense to set RequiredFunctionName to null and not the first tool if multiple tools are specified, that would make providers that don't support multiple tools to fallback on the ANY mode.

### API Usage

```csharp
var getCurrentLocationFunc = AIFunctionFactory.Create(() => "Paris", name: "get_current_location");
var getTemperatureFunc = AIFunctionFactory.Create((string location) => 15f, name: "get_temperature");

var response = await chatClient.GetResponseAsync(
"What's the weather like today?",
new ChatOptions
{
Tools = [getCurrentLocationFunc, getTemperatureFunc],
ToolMode = ChatToolMode.RequireAnyOf([getCurrentLocationFunc.Name, getTemperatureFunc.Name])
});
```

### Alternative Designs

- Rename RequireAnyOf to RequireSpecific as it would be closer to the provider implementations but I think the former is clearer
- Use a list of AITool instead of a list of string because of https://github.com/dotnet/extensions/issues/6288
- Not providing this feature at all. It seems like this feature is not very different from ChatToolMode.RequireAny and filtering the tools.

### Risks

- Some providers don't support the feature
- OpenAI .NET doesn't seem to support it in their client even if it's supported by OpenAI
- The two properties RequiredFunctionName + RequiredFunctionNames can be confusing.