Minimal API friendly version of `MapWhen`

[aelij]

Is there an existing issue for this?

• I have searched the existing issues

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

When using MapWhen we get a lot of nesting:

app.MapWhen(ctx => ..., appBuilder =>
{
    appBuilder.UseRouting();
    appBuilder.UseEndpoints(routeBuilder => 
    {
         routeBuilder.MapGet(...);
    });
}

app.MapWhen(ctx => ..., appBuilder =>
{
    appBuilder.UseRouting();
    appBuilder.UseCors();
    appBuilder.UseEndpoints(routeBuilder => 
    {
         routeBuilder.MapPost(...);
    });
}

Describe the solution you'd like

It would be nice if we had a top-level API, for example:

var app1 = app.MapBranch(ctx => ...);
app1.UseRouting();
app1.MapGet(...);


var app2 = app.MapBranch(ctx => ...);
app2.UseRouting();
app2.UseCors();
app2.MapPost(...);

MapBranch can return a new type implementing both IApplicationBuilder and IRouteEndpointBuilder.

Additional context

No response