Allow WebApplication.CreateEmptyBuilder to have host configuration provided using ConfigurationManager

### 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.

A am trying to use `WebApplication.CreateEmptyBuilder` but my host configuration is loaded from multiple sources (files), but there is no way to provide host configuration to `WebApplication`, even though the inner `HostApplicationBuilder` accepts a `ConfigurationManager` through its options.
Current design forces me to copy the contents of the `WebApplicationBuilder` class into my project, just to be able to forward my 'external' configuration.

### Describe the solution you'd like

Add `ConfigurationManager? Configuration` property to the `WebApplicationOptions` that can be used to seed configuration by users.

### Additional context

_No response_

Start by tracing WebApplication.CreateEmptyBuilder and WebApplicationOptions into the inner HostApplicationBuilder, noting how ConfigurationManager is currently accepted there. Check how WebApplicationBuilder forwards its options and determine where the new configuration value should be seeded. Done means external host configuration can be supplied without copying WebApplicationBuilder, while preserving existing behavior when it is absent.

Written by the indexing model from the issue text.