Replace SKContext as Function/Kernel result type with FunctionResult and KernelResult models

Context and Problem Statement

Methods function.InvokeAsync and kernel.RunAsync return SKContext as result type. This has several problems:

1. SKContext contains property Result, which is string. Based on that, it's not possible to return complex type or implement streaming capability in Kernel.
2. SKContext contains property ModelResults, which is coupled to LLM-specific logic, so it's only applicable to semantic functions in specific cases.
3. SKContext as a mechanism of passing information between functions in pipeline should be internal implementation. Caller of Kernel should provide input/request and receive some result, but not SKContext.
4. SKContext contains information related to the last executed function without a way to access information about specific function in pipeline.

Decision Drivers

1. Kernel should be able to return complex type as well as support streaming capability.
2. Kernel should be able to return data related to function execution (e.g. amount of tokens used) in a way, when it's not coupled to AI logic.
3. SKContext should work as internal mechanism of passing information between functions.
4. There should be a way how to differentiate function result from kernel result, since these entities are different by nature and may contain different set of properties in the future.
5. The possibility to access specific function result in the middle of pipeline will provide more insights to the users how their functions performed.

Considered Options

1. Use dynamic as return type - this option provides some flexibility, but on the other hand removes strong typing, which is preferred option in .NET world. Also, there will be no way how to differentiate function result from Kernel result.
2. Define new types - FunctionResult and KernelResult - chosen approach.

Decision Outcome

New FunctionResult and KernelResult return types should cover scenarios like returning complex types from functions, supporting streaming and possibility to access result of each function separately.

Complex Types and Streaming

For complex types and streaming, property object Value will be defined in FunctionResult to store single function result, and in KernelResult to store result from last function in execution pipeline. For better usability, generic method GetValue<T> will allow to cast object Value to specific type.

Examples:

// string
var text = (await kernel.RunAsync(function)).GetValue<string>();

// complex type
var myComplexType = (await kernel.RunAsync(function)).GetValue<MyComplexType>();

// streaming
var results = (await kernel.RunAsync(function)).GetValue<IAsyncEnumerable<int>>();

await foreach (var result in results)
{
    Console.WriteLine(result);
}

When FunctionResult/KernelResult will store TypeA and caller will try to cast it to TypeB - in this case InvalidCastException will be thrown with details about types. This will provide some information to the caller which type should be used for casting.

Metadata

To return additional information related to function execution - property Dictionary<string, object> Metadata will be added to FunctionResult. This will allow to pass any kind of information to the caller, which should provide some insights how function performed (e.g. amount of tokens used, AI model response etc.)

Examples:

var functionResult = await function.InvokeAsync(context);
Console.WriteLine(functionResult.Metadata["MyInfo"]);

Multiple function results

KernelResult will contain collection of function results - IReadOnlyCollection<FunctionResult> FunctionResults. This will allow to get specific function result from KernelResult. Properties FunctionName and PluginName in FunctionResult will help to get specific function from collection.

Example:

var kernelResult = await kernel.RunAsync(function1, function2, function3);

var functionResult2 = kernelResult.FunctionResults.First(l => l.FunctionName == "Function2" && l.PluginName == "MyPlugin");

Assert.Equal("Result2", functionResult2.GetValue<string>());