.NET: Consider improving the discoverability of required functions

@westey-m is already working on this.

Since Sep 17, 2025.

Currently the code looks like this

```csharp
// Create the chat client and agent.
// Note that we are wrapping the function tool with ApprovalRequiredAIFunction to require user approval before invoking it.
AIAgent agent = new AzureOpenAIClient(
new Uri(endpoint),
new AzureCliCredential())
.GetChatClient(deploymentName)
.CreateAIAgent(instructions: "You are a helpful assistant", tools: [new ApprovalRequiredAIFunction(AIFunctionFactory.Create(GetWeather))]);
```

Consider add an extension method e.g. `AsRequiredFunction()` so developers can more easily discover this

`AIFunctionFactory.Create(GetWeather).AsRequiredFunction()`

Or add a new method to `AIFunctionFactory`

`AIFunctionFactory.CreateRequiredFunction(GetWeather)`