Proposal: UseSystemCommandLine extension for IHostBuilder

Hi team,

while building .NET apps (console, WPF, Windows-Service) on top of System.CommandLine, I often combine it with Microsoft.Extensions.Hosting.
Today each app must:
• parse args manually,
• inject ParseResult into DI,
• handle --help / validation,
• wire optional post-parse configuration.

I propose a small host extension:
```
Host.CreateDefaultBuilder(args)
.UseSystemCommandLine(rootCommand, args, (parseResult, services) =>
{
// optional post-parse configuration
});
````

Benefits:
• Unified startup for console, WPF and service apps
• Single parse before building the host
• DI-friendly access to ParseResult or a typed accessor

This would live as an additive helper, so no breaking change.

Would you consider such an addition?
I’m happy to draft a PR if the idea fits the roadmap.

Thanks!