Add progress callback

[869570967]

1. Expose the progress callback of Downloader.Download step by step to external interfaces.
2. Add an Action progress parameter to RunCommand, and also add an Action progress parameter to PipInstallModule, which calls it.

Purpose: When calling functions like await Installer.TryInstallPip(); and await Installer.PipInstallModule("torch", progress: progress); sometimes, due to network issues or very large packages, the program has to wait a long time without any progress feedback. Simply showing a loading spinner isn’t ideal, so here we add a progress callback parameter to make it easier to display the current progress in real time.

PS: The code was implemented using AI, and self-testing shows no issues so far. Here's my test code—you can also try it out to see if it works. If there are no problems, it can be merged and published to NuGet (pre-release is fine too). I'm a bit pressed to use it. Thanks a lot!

`using Python.Included;

namespace PythonDemo
{
internal class Program
{
static async Task Main(string[] args)
{
Installer.LogMessage += Installer_LogMessage;
//Console.WriteLine("Hello, World!");
await Installer.SetupPython();
await Installer.TryInstallPip();
Action progress = p =>
{
Console.WriteLine(p);
};
await Installer.PipInstallModule("torch", progress: progress);
}

    private static void Installer_LogMessage(string obj)
    {
        Console.WriteLine(obj);
    }
}

}
`

[869570967]

Here are my test results: