SpaProxyMiddleware Launch Page

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

I am trying to create a custom spa launch page, but the SpaProxyMiddleware doesn't allow for it. The middleware is using a privately generated html string, and doesn't provide a way to override.

### Describe the solution you'd like

Given that the 'GenerateSpaLaunchPage' method takes a parameter for 'SpaDevelopmentServerOptions', surely it would be easy enough to allow an option for our own custom html string, and not get stuck with a launch page that doesn't match the existing site at all.

Perhaps something like:
```cs
await writer.WriteAsync(_options.Value.LaunchHtml ?? GenerateSpaLaunchPage(_options.Value));
```

### Additional context

https://github.com/dotnet/aspnetcore/blob/main/src/Middleware/Spa/SpaProxy/src/SpaProxyMiddleware.cs line 63