Add support for custom delegates as SKFunctions

[dluc]

Motivation and Context

In some scenarios like RPC and OpenAPI it can be useful turning native code into SK Functions allowing to pass signature details (name, description, params) without the use of SK* attributes. For example, loading these details from OpenAPI docs or config files, rather than using reflection.

Description

Add a new FromNativeFunction factory method to SKFunction to pass a custom delegate. The custom delegate must use one of the supported signatures, but doesn't need the "SK*[...]" attributes.

Example:

async Task<SKContext> MyFunctionAsync(SKContext context)
{
    await Task.Delay(0);
    return context;
}

ISKFunction function = SKFunction.FromNativeFunction(
            nativeFunction: MyFunctionAsync,
            skillName: "Test",
            functionName: "HelloWorld"
            description: "This is a test");

[dluc]

The PR is ready to be merged.

One of the issues fixed here has been addressed also in #900 so the remaining part is fixing how native function signatures are handled reusing the same code used for native skills.